Footswitch cables are wired usually very simply. Nevertheless you should not try using a Y-splitter here. There are two reasons:
a) you don't know the behavior of the open switch on the amplifiers. If both amplifiers have the exact same circuitry, connecting their "open" switch signals should not cause problems but nobody gives a guarantee for that. If you have different amplifier models, all bets are open.
b) you'll get a ground loop. Those are, like, really bad news.
That does not mean that you are out of options. Take a look inside of the foot switch: do the switches have an unused set of connectors? If so, you can root out another pair of (ground-isolated!) sockets from there. This will avoid both the ground loop as well as the "open-together" wiring problem.
Other than that, you'll have to wire your own "relais box" "talking" to the foot switch on one side and (via isolated and separate contacts) to the amplifiers on the other side. Or you build a box of your own. Or you just change the foot switches in your existing foot box for double-contact ones and then use the separate ground-isolated socket approach.