So, I have a number of thoughts on this mechanic, and designed several additional cards. I've been slammed at work, which is why I didn't post any of this sooner, and I'm actually going to break this into multiple posts.
One big question I had about this mechanic, in terms of design for it, is how strong is the capacity to combine cards (and, in particular, to combine Action cards). My first impression is that combining Action cards is incredibly strong. It is not hard to think of 2 card combinations that are extremely powerful. There are obvious examples:
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Smithy | + | Village |
and some less obvious, but very strong examples, like
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Tactician | + | Watchtower |
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Highwayman | + | Tide Pools |
Even if you put some kind of a cumulative price limit on the combo, you can still get incredibly powerful combos with cheap cards, like...
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Moat | + | Scheme |
...which is a double-Lab you get every turn, and gives you complete protection from attacks (that is even stronger than Champion as, unlike Champion, you have the option to hold back Moat + Scheme when you want to be affected by the attack), or...
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Courtyard | + | Native Village |
... a net-LC which thins from your deck (while keeping for scoring purposes) a card each turn (by topdecking it, then setting it aside with NV).
Now, these are specific combos, but some obvious patterns emerge: pairing non-terminal cards (especially cantrips) with terminal cards (especially terminal draw), you get the added strength that terminal cards provide with no risk of a terminal collision. Combining cards that move themselves with cards that you benefit from always/usually having can allow you to have that other card far more than you otherwise would.
Taking a step back to think more analytically, there are three ways in which combining Action cards provides a huge boost to their power:
Two-for-one: When you think about Action cards and what they do, one useful tool is to imagine that every Action card (when played normally from your hand) has a hidden “-1 Card, -1 Action” on it; you play the card from your hand (reducing your handsize by one) and spend an Action to play the card. Thus to the extent a card is a “cantrip” (e.g. gives “+1 Card, +1 Action”) it leaves you in the same position as if you hadn’t played the card, and anything you get in excess of that (+1 Action from Village, +1 Card from Lab, +$1 from Peddler) can be said to be the “net” effect of the card. When you combine two Action cards, the hidden effect of playing both of them from your hand goes from being “-2 Card, -2 Actions” to being a regular “-1 Card, -1 Action.” That means the net-effect of combining any two Action cards is +1 Card, +1 Action, or a Lost City. Lost City is underpriced at $5 (which is why it gives you a penalty in the form of a bonus for your opponents), so it should almost always cost at least $5 to combine two Action cards (this is an oversimplification, because the LC you get from combining cards can only be played together with those 2 cards, as opposed to an LC that goes into your deck and can combine with any cards for extra Cards/Actions).
Collisions – Once two cards are combined, they will always be played together, in order. This can be incredibly powerful. The most obvious examples are to combine terminal cards (especially terminal draw) with non-terminal cards. Because of the hidden LC, they don’t even have to be villages, anything that gives +1 Action makes the whole package non-terminal, and quickly turns into an incredibly powerful card.
Following cards played not from your hand – the last advantage is a somewhat more obscure one, and only helps in a limited number of contexts. In almost every other case, when you play an Action card, you have to play it from your hand. That means it has to be in your hand immediately before you play it. By combining cards, you can play subsequent cards not from your hand. This means that even if the first card empties your hand (i.e. with Tactician) or works better without Action cards in your hand (e.g. Shanty Town), you can play them, and play another Action right after them. The only way to do this will official cards is with Golem (which gives you almost no control over what the 2 cards will be) with a Throne variant and a Way (which only lets you play the Way and the card you Throne as the different options), and by setting up multiple cards to be played at the start of your turn (using Delay, Way of the Turtle, certain Durations, etc.) which is limited both in needing those cards to enable it, and in happening at the start of your turn (so you can't first play the cards you have, which is what you'd want to do for, say, Tactician + DtX).
All of this suggests that combining cards, especially Action cards, needs to be subject to some important limitations. It either needs to be expensive (at least $5), or it what is being combined needs to be subject to some important limitations. I tried to keep that in mind when I was designing my cards.