Hi guys.

I've been developing a turn-based game, which has both single player and (local) multiplayer options. The gameplay is largely similar between the two apart from the fact that CPU controls one of the turns in single player.
Is it wise to make a separate Android activity for both options or make one activity which handles both via booleans etc?

Thanks in advance

I would keep it as one.

Creating a second will add significant complexity and therefore add bugs, but you are only getting a tiny benefit of trying to simplify multiplayer.

It would be easier and less error-prone to design your game core such that you can create multiple object instances, one instance per player.