This is way too detailed, but describes how barrels behave while a shot is being fired: http://www.the-long-family.com/shock%20wave%20theory%20summa...

Based on this, it seems like multiple barrels probably wouldn't affect each other in different ways, leading to shot consistency as long as there aren't significant differences in ignition time or velocity.

Almost forgot: while not simultaneous, the AN-94 did not have issues with ultra-fast RPM in one barrel (https://en.wikipedia.org/wiki/AN-94).