This has probably been more a challenge for me than you, so much analysis to do with split piles! Well done all, there are some great designs here. I feel I didn't have the time to properly, fully analyse each one but they're all considered.
So, in looking for good use of the split pile mechanic, I was wanting to see convincing reasons for players to contest the pile, good involvement of the timing each card appears, interesting manipulation of the pile, a definite synergy between the two cards that isn't independent, and all the other things making the Empires split piles meaningful.
Dragonfly/Bug Collector
Not bad, this one. Cursing goes well in a split pile. The synergy between the cards is sound yet not so close that the pile can't interact with others; possibly a bit too complete a big money strategy. Dragonfly is in a tough spot price wise though, maybe if were a bit cheaper and weaker it could let players contest them easier, and let the crows fly sooner.
Investment/Banker
There are good design elements in this pile. Banker needs at least 2 Investments without other Durations, which creates some competition for the Investments. But as holunder said himself, there are risks with how strong it is. Investment hoards Coffers, and 3 Investments could be a win condition. Another snag is Investment feels like it should be an Action as it doesn't give immediate benefits.
Wages/Day Labourer
This one was certainly popular. It has a great first impression and follows the principles of a good split pile very well, but I fear the pile synergy is a bit too close and too strong. +2 handsize +1 Action +$1 across 2 cards, better than 2 labs; they each cost $5, whereas these are just $2 and $4.
Magic Crown/Vampire Queen
The Crowns are certainly hot competition, possibly they're worth $4 as they're close to a peddler but they're good fun at $3. Then the Queen; the Action bit is bad, but lets her Throne herself. It's convincing, but possibly Queen is a bit weak? Could she be $4 even?
Cobalt/Kobold
Cobalt very nicely fuels the Kobold, and it makes a fun race to buy the first Kobold like Gladiator to Fortune. There is the gift of Copper trashing for opponents though, which could be minor as it comes from hand; and the first Silvers are nerfed with their money density being the same as Copper, so less likely bought, but sometimes opening Silver has to be done to hit $5 and it adds a +Buy. Yes, this is promising.
Contract/Bookkeeper
Very similar to Wages/Day Labourer. It may not have as exciting a first impression, but it's safer. Contract will probably be contested more than Wages, but I agree that it's a bit cheap, because a Treasure giving a Villager is very, very good with terminal draw. Also means Bookkeeper being draw to X is a good move, you only get the crazy non-terminal draw once unless you work for more. Sound design.
Surplus/Glutton
There might be something going for the basic idea of this pile, but these cards are rather weak. Surplus is a bit steep for what it does, and Glutton is a very expensive Peddler only doing better than one a very few times. Golds to hand aren't that good.
Pheasant/Chef
Great payload potential but with deceptive snags, mainly with how Chef loves Gold and Platinum but Pheasants aren't going to get them so it'll be awhile before you do. Nice, but double Pheasant opening could be automatic in some games, almost like Quarry at $3 with an extra Buy; my main concern is the strength of this in building speed, despite the later game setbacks.
King's Shilling/Garrison
To trash Garrison to Shilling you have to not play it, so the synergy is weak. Using Garrison as a gainer is awkward as it comes in late. And I feel Death Cart does what Shilling tries to do much better.
Coin Press/Smelter
A few issues with this, the most prominent being that 2 Lost Cities, 2 Smelters and 1 other cantrip can play infinitely. Train the Lost Cities for infinite $ and Buys, gain the Supply.
Candle/Ghost Hunt
Similar to Magic Crown/Vampire Queen. The synergy is weaker, since the discard pile is unreliable. This whole pile seems to revolve around the King's Court bit on Hunt, which is non-terminal but has a lot setting it back; it's on a Duration so it's every other turn, sometimes you have to avoid playing the Action this turn to set it aside, and candles are Treasures getting in the way.
Mould/Smelter
Players will only want to buy Moulds for their +Buys, situationally for making into Gold. So Smelter will rarely appear, and when it does the best it usually gives for payload is Woodcutter, trashing Coppers or Moulds that could have been played for $2. And as a trasher it comes in too late.
Horse/Paddock
When Treasures are a relevant strategy, Horse can make for a competitive top card, but you have to be even more careful than you do with Lurker because nobody minds a Gold. Possibly players would rather buy a Silver to be safe and reliable. Paddock is strong and worth emptying horses for, maybe too good with big money decks?
Writ of Credit/Margin Trader
Very mathematical. Looking at why and when you'd buy each card, Credit at the start is a Copper trasher but compares unfavorably to most other trashers, so if they're in the game you'd ignore Credit at the start. To get tokens from it, you'd want it later on and have the cards to trash to it; Silvers I suppose are best, you can pay the debt off easily. So, I don't think Trader will appear very often, and it's a bit too dependent on Credit to work; if you have no debt it's nearly useless. The bottom cards of the official split piles can be used fine without the top. So this premise doesn't really suit a split pile.
Goldsmith/Medallion
This is an interesting format to be sure. The viability of Medallion is put into question with Villagers being so good with Goldsmith's draw, and with a Copper, Silver and Gold in hand you can buy a Gold (I assume the reveal is from hand, it isn't actually stated) so a lot of Villagers are needed to make it a sensible buy. It might overall depend on other sources of Villagers to work. Goldsmith by itself could be a nice card if it's balanced.
Tool/Anvil
Tool looks very scary. Line two up to trash 3 cards, then after trashing they make themselves lesser Fool's Golds that line up very easily. And they cheapen Anvils. I see that this pile is very dominant very often, all you need is a Village and this is practically a complete strategy.
Gilt/Lily
Definitely too heavily nerfed. Gilt is very slow Copper trashing, and only when there's no kingdom $2 pile, so other trashers are preferred. They empty very slowly, if at all, revealing Salt the Earth in portrait form; it's rather unimpressive as a generic source of VP. You'd get this if you're comfortably ahead, but will you be if you've played a very slow game with Gilts?
Timberland/Treant
Number of copies sensible, but there's so much going on here I hope I have it all right. It all hinges around the cost increase on Treant. Timberland might be wanted enough to reveal it, change a copper to a $3 with 1VP. When revealed you can get a $5 (which can't be Duchy), unless you return Timberland to hide it again so you get a $3. All quite neat, but is there synergy between Timberland's gaining and Treant's wanting nice victories? Not really, the only link I see is liking sifting, which may not be present. And Tunnel, the same. Balance? Hard to tell, but Treant doesn't look very safe.
Magic Beans/Beanstalk
I'm not sure Beans will be bought enough to make Beanstalk appear that often, including the ones Beanstalk returns.
Shortlist: Dragonfly, Magic Crown, Cobalt, Contract, Pheasant.
So close between these 5, but I have to tell myself to stop considering them sometime. Upon further reflection…
Winner: Cobalt/Kobold by hypercube.
Runner-up: Pheasant/Chef by Fragasnap.
Congratulations on a challenge well answered.