According to kevin :
Imbalance won't be a problem with a twopole GFCI breaker. Confusion
comes from this statement:
The reason for this is simple: the hot and neutral current though
_each_ GFCI must exactly balance.

This is true, but not the way the GFCI works.

Actually it is. The whole point of the toroidal coil is to detect
when they imbalance. With a two pole GFCI, the coil's just wired
differently so that they detect when the neutral current isn't
the difference between the two hot currents.

If you tried to do a multi-wire branch circuit with two single pole
GFCIs, the toroids (plural) aren't wired right to detect imbalance
properly. So they be tripped all the time.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.