Did I already make a thread about this? I forgot
You can approximate the probablities by letting one character use the full d20 as per normal, and then just look at the last digit to see whether or not the monster succeeds.
You kinda might need a lookup table for it; crossrefernce the monster's to hit bonus with the target's AC. If you're already using a version of the game where that's the norm (like RC).
For example let's say Alice hits the monster on a 13+, and the monster hits Alice on a (just looking at the last digit) 6-.
It becomes similar to Hack & Slash in DW. I'm not eager to switch to this, it was more a thought experiment.
Obv the probs don't map exactly because you're replacing 2d20 with 1d20. Buuut it actually got a lot closer than I expected.