So it first determines if Sim 1 impregnates Sim 2 (25%), then if Sim 2 impregnates Sim 1 (25%). So the chances are like having two WooHoos in a row...
Maybe decreasing the chance for a "Both" Sim to get pregnant when having sex with another "Both" Sim could push the probability of a child regardless of impregnator and impregnated down to 25%...
It's math time. I'll have to think, but I think that currently a "Both" couple has a higher chance of one getting pregnant than any other couple. I'll see if I can come up with a solution. Unfortunately I can only comment here because the other comment section is locked.
Edit: I got the solution. Whenever two "Both" Sims have sex, the fertility chance (the one you can set) has to be calculated according to this formula
sqrt( - sqrt( 1 - 2 * f^2 ) + 1 ) / sqrt( 2 )
with f being the set fertility chance (let's say 50%). Then it can be passed to the calculation of pregnancy.
The result is 38.27%, so the chance of Sim 1 impregnating Sim 2 is 38.27% * 38.27% = 14.64%. That's also the chance of Sim 2 impregnating Sim 1. As such, the chance of the WooHoo resulting in a pregnancy can be determined using binomial distribution f ( 1, 2, 0.1464) = 25%. That makes a pregnancy as likely as if it was a non-both WooHoo.
Edit 2: That method only works for simple pregnancy mode. I still have to come up with a general solution.