I alrady said, that the chance is pretty low, but let's speculate a bit.
If during Gallienus reign, the exercitus moesiae usurps, Gallienus' opportunities are very limited. The exercitus pannoniae is all, he could use against the usurper, because the central units in northern Italy had to guard the alps, to avoid, that Postumus is marching on Rome.
The moesian army is perhaps not strong enough to attack, busy with Goths or has to retreat after a first tied battle. The moesian army controls parts of Asia minor, which should be sufficient to supply it.
Now assume another even more unlikely event: the exercitus africae usurps as well. Way to weak, in order to march on Rome. But able to hold Africa against a weakened Gallienus. Well, there is a reason, why african armies did not usurp that often, and if, they often did it under weird circumstances. Most probably Gallienus is dead, if he cannot reconquer Africa immediately, in order to guarantee corn supply. And as described above, he can't.
So we got 5 roman empires and 4 of the 5 rulers call themself the one and only legitimate emperor. However, the next emperor with balls, in any of these sub-empires, will try to beat his neighbour. And the winner takes it all. As mentioned above, separate roman empires were a strong violation of the roman mindset. Nobody would understand, why not to goal for a united empire. There is always a very strong trend for reunification in the roman empire, which es extremely hard to butterfly away.