I've never seen them go at it on their own. The closest is that they both relax in the same bed, then you just tell them to go at it. There is also a short video the first time they go at it Sims can't get pregnant when the house is full, but after your first victim you'll know when someone is.

I hate killing Sims with Starvation because they eat all of your food as ghosts, and it's annoying =/