They are done, of course!
Sbs split jumps are routinely done in Pairs skating, and there is even a Pair stag jump done by teams (remember teams from the 70's and 80's).
The problem is that the split jumps do not get the credit they used to get, so we see less and less of it at the elite level. Also, it is hard to get the same flexibility from the man and the woman, so the lines are hard to match up.
You will see sbs splits in the lower level Pairs events.