they will both use the stairs, but they won't be talking nor will they stop halfway. Be sure to leave enough spare squares at the top, as in don't cover all available tiles with floor. That used to be a thing in Sims2 and is still a thing with Sims 3 most of the time, like many of the pre-built houses in Sims2 only left 4 squares uncovered at the top and the sims would complain that they can't get down them, a minimum of 6 squares ensured they didn't do that, and they would complain half the time if it was a single set of stairs, so I used to try to either have a double set or have a second single one at the other end of the hallway.