Ugh, I am so sorry. I didn't notice this was in the AI section as I just found it in new posts. I just don't use AI enough to answer your questions. I just use it one doc at a time to make something to use in another program, usually PS. It being Adobe, I imagine the file options are similar. So my answer below is for Photoshop. Maybe someone else can help out . . . or I can open AI and see ... but I don't have time tonight. Interesting that you ask if your project is meant for PS. I can't tell as I'm not sure what you're doing, but if it would work there, then maybe so.
They are both separate projects. I saved the final work of the one I need added but I don't know if its possible to throw it into my other work.
Still confused here. I will answer what I think you're asking . . .
a couple ways:
At anytime in Photoshop, you can have multiple documents open. If you saved and closed and now have a different doc open, open the saved one again. If you want to bring over all the layers, you can move them one at a time. If you can't get the move tool to work for you for whatever reason, use copy and paste. First select all, ctl/cmd +A, then select the layer and copy. Go over to the other document and paste.
There is also a Place command under the file menu. when your doc is open go to file>place, pick your desired file and once it appears, hit enter. It will place a flattened version of a multi-layer psd, like if it were saved as a jpeg. That way you don't even have to open the saved doc, just place.