Sound gaps? There shouldn't ever be any. In a well-crafted orchestration something is always going on in other books so that you don't "hear" when players are switching. Who doubles on what and when is something carefully charted out.
Occasionally, two instruments at a time ARE called for, lol. Percussionists, of course, do this constantly. It's not uncommon for a keyboard player to be playing 2 synths at the same time (the original Little Shop had 3 synths layered on a stand, and the player often needed to have hands on 2 keyboards at a time). There's also a moment in West Side Story (before synths were a thing) where the pianist plays with the left hand on piano and the right hand on celeste.