If you're flying a metamidion you definitely want three engineers.
If you're flying a mid range pyramidion built around the hades (often paired with a metamidion) you need gunner to be able to alter the arming range in order to keep up with and support your ally, if you have an extremely good gungineer capable of keeping both front guns and the balloon sprayed and buffed in combat you don't loose anything except repair capability in tank-mode, fortunately being a mid range build you are less likely to be forced into tank-mode (your close range ally is a larger immediate threat to be focused).