temporary loss of FPS with gen switches

I have noticed that. just after having started both engines, whenever i select both generator switches on the overhead panel so to take power from the engines instead of from the APU i will suffer from a little freeze followed by approx. a 30 second period of significant loss of FPS. This only happens when i swith both on at the same time. When I do this one-by-one (in other words: after the first engine has finished starting and is stable i apply the gen switch for THAT engine and THEN continue with starting the 2nd engine, wait for that to stabilize and THEN turn on the generator switch for THAT engine) all is OK. anyone recognizes this?