I have Krark’s Thumb (I think that’s the name) and the newly spoiled Two-Headed Giant out. What’re the odds of me getting, say, menace from its triggered ability? Is there any interaction here at all?
There is absolutely an interaction here, and quite a good one.
First of all, let’s explain exactly how the interaction works here: Instead of flipping two coins for Two-Headed Giant when it attacks, you’ll flip two pairs of coins instead. Then you’ll choose one coin from each of the two pairs and those two chosen coins will determine what the trigger does. The important thing to note here is that you don’t just get to flip four coins and choose any two of them that you want.
In general, you go from having a 50% chance of Two-Headed Giant gaining an ability to an 87.5% chance. 25% of results even let you choose between the two abilities!
As for the odds of getting a specific result, the following chart shows the possibile results from each combination of two pairs of coin flips.
You have a 56.25% chance of getting a specific ability, either double strike or menace, from two pairs of coins flipped this way. You’re only ever forced in a specific ability 12.5% of the time when the coins either end up all heads or all tails. And you only ever miss out on an ability in the 12.5% of cases where one pair of coins is both heads and the other pair of coins is both tails.