You can just square your stance, and have both weapons roughly in range though.
Getting any meaningful swing with both at roughly the same time is however just awkward - one can take a swing in one direction, step onto one leg, shift his body weight etc. in one direction, attempt at two at the same time will be rather counterproductive, from obvious reasons.
I guess that's why people were attempting double rapiers - with weapons of such strictly thrusting dynamics, one could try to make it work.
Case of rapiers was described in manuals from all around the Italy, so this wasn't some very isolated idea.My understanding is that Florentine duelists usually managed to die in the process of killing their opponent. One can imagine how that wouldn't really catch on.
Probably used mostly by people who were desperate for adrenaline still.