Also, tangent, I realized there's no need to include Banes in probability unless Young Witch is part of the combo with a $2 or $3 card. If your 2 cards don't cost $2 or $3, then the topic is moot; we don't care if there's a Bane, since it doesn't change the probability of the combo occurring. If one or more of the cards cost $2 or $3, then if one of them is the Bane, then that's equivalent to replacing Young Witch with that card, in terms of probability. So for most combos, we don't care about Banes.
If Young Witch is in the combo with a $2 or $3 card - let's say YW/Tunnel - things get a little more interesting. Allowing for a Bane increases the possibility of the combo occuring ever so marginally. Since Tunnel not being the Bane and being the Bane are two mutually exclusively events, we can deal with them separately, and then later add them. i.e. Prob(Tunnel != Bane) + Prob(Tunnel == Bane) = Desired probability
So our normal 2-card probability is 9/4223 (9/5546 with Adventures). Now, a probability is just the number of combinations *with* our desired event divided by the total possible combinations. The number of combinations with Tunnel as Bane are going to be using n=204 (we've removed YW and Tunnel from our set), k=9 (YW is guaranteed), d=0 (we want the combinations WITHOUT Tunnel) - each of these has one combination with Tunnel as Bane. This is 204!/(9!195!). We now have 205!/(10!195!)+50*205!/(9!196!) possible combinations [since there are (before Adventures) 55 $2/$3 cards, leaving us with between 45 and 55 options to choose from for Banes, depending on how many $2 and $3 cards are in the Kingdom, which I've averaged to 50 since this number is going to be very small compared to the other one anyway].
Thus:
Probability of YW/$2-3 card combo = 9/4223 + 1/(205/10 + 50*205/196) = 9/4223 + 1/(20.5+ 52.3) = 66.98/4223 = 1/63.04, which is marginally larger than the normal 2-card probability of 1/469.22.
Of course, this will depend *slightly* on your choice of $2-$3 card averaging constant.
Never mind, did this wrong, head hurts