Certainly that's true if I have infinite capital, but if these markets require participants to have infinite capital in order to work, we've got a problem. If I have finite capital, then any money I put on the factory is money I can't put on the mill, and that's losing value.
Actually, if the options are mutually exclusive and bets on the losing option get voided, there's no reason to forbid you from betting on both options with the same money, is there? Only one bet will stand.
For that matter, anyone could safely lend you X, where X is what you already bet on A, for the purpose of betting on B. One way or another you'll get X back in voided bet money, so you're a perfectly safe borrower.
Typically in a betting market, you can continue to buy and sell your shares after placing your bet, so if the market moves and you now think that A is overpriced, you can sell some of your shares and lock in profit. It's not entirely clear to me how you make this work if your investment in A and B is with mirrored funds. If there's a way to make it work, it certainly seems like a step in the right direction.
If you have a budget constraint, it's rational for you to buy argmax(true(A) - market_value(A), true(B) - market_value(B)), which is exactly the Pareto-efficient behavior.
That's where I disagree. Your expected value on buying A is (probability A is implemented) * (true(A) - market_value(A), and similarly for B, because your receive zero return if the thing you bet on is not implemented. Thus, even if A is badly mispriced, you may not want to buy it if it has very low probability of being implemented.
