I'm gonna be agreeing with the fabulous Fabian on this one. BM/BM is just plain silly. I actually like having the card in my deck, as I tend to perform better at building 20 card engines than doing a simple FV/Torturer chain (I mean I can execute it properly, but so can anyone else. Therefore it's just a question of shuffle luck.) However, if there is an engine on the board, opening BM/BM hoping for that lucky turn 3/4 ambassador will put you waaay to far behind in building the engine. By the time you start handing them estates and coppers, they will have that engine up and running. And that's not the only problem. You got the ambassador? Great. Now you have no +draw, no +actions, and a card that's going to hurt your economy rather than help it. Turn 5 you end up drawing BM/BM/Amb/copper/copper. Well you could give him copper, but at that point your opponent is likely already hitting 5 and 6 for those strong actions the chain relies on. And if he isn't, that means he already has a trasher that will make ambassador irrelevant. Well, there's also the option that he's running a strategy where it won't matter (like HT/Gardens). Oh, and your turns 6 and 7 will suck now. And in almost any situation you really need something from the BM deck, it's better to grab a support card than another BM. A young witch that will cycle your deck and add curses slowing your opponent down (which helps if your opponent wants the BM card too). A wishing well or a courtyard that will get your BM faster as well as the BMed card. A swindler that turns his BM cards into something useless. Something like that. Sure, there may be a board where BM/BM is a good opening. But certainly not in a situation where ambassador is good.
(Either way I'd much rather take something like masquerade, tournament, tactician, tunnel, vault, watchtower, menagerie, scheme, mountebanke, or goons from the BM deck. These cards are all something that you can add to the deck midgame and immediately make your deck better. If you are using BM, you need quick cards to make up for the time you lost.)