I’m working on something that different players alternating between compound (12/8) and simple (4/4) simultaneously. Is there a way I can input the time signature to look like 12/8 (4/4) and be able to write in duple or triple? Or do I need to notate it in triple with duple indications in those parts that are duple, or notate it in duple and have a ton of triplet indications?
Do you mean that different instruments in the group will have 12/8 simultaneously with 4/4, or that the whole ensemble will alternate between 12/8 and 4/4? It makes a difference as to how to approach it in Dorico.
Different instruments will have 12/8 and 4/4 simultaneously. And those groups will change; a section that had 4/4 mace move to 12/8 later in the piece and vice versa.
See this thread (or any one of a number of similar ones) that address this exact situation.
There’s also a Scoring Notes article that seems very relevant to your situation: