Again, this depends on the feed tray and how its loaded. Does not matter how many guns there are. ROF is a constant.
The Flakvierling 38 has no feed tray. It's magazine fed. Once the 20-round magazine is empty, it's removed and a new one is inserted. As a single barrel fired at 480 rounds/minute, the magazine was empty after 2.5 seconds of continous fire. One loader had to reload 2 barrels each. So firing all four barrels at once (by pushing down both both pedals) meant a short stop after 2.5 seconds, during which the gunners had to insert two new magazines. Only then the gunner could continue firing.
If the gunner chose to fire only two barrels at once, the loaders were able to reaload the one currently not firing. Only that way continous sutained fire was possible.
And that's why you can read "practical rate of fire: 800 rnds/minute" in most data sheets. That's the constant ROF. The 1800 rounds/minute is the theoretical maximum ROF which can't be sustained.
Our AH2 Wirbel is shooting at a continous ROF of 1800 rds/min.
That's the same gun the Wirbelwind is using. See the magazines.