I am not sure about merging two games, but the current version of bitsy supports changing the avatar between rooms within a single game. Aside from your main avatar, you can designate any sprite you have made to be the avatar for a particular room by clicking on the avatar option at the bottom of the room editor (scroll down in the room editor window if the button isn't visible). Especially if you're new to coding and bitsy, I recommend using the built-in tool first because it's very simple and doesn't require digging into the code.