Sorry if this thread has been made many times already, or this question has been answered elsewhere, but I can't seem to find a solid answer

Simple few questions. I have trinity, and my friend went and bought rebellion. We want to play together, but because of the silly decision to make rebellion a stand alone (feels like a money grub to me, but that is just me, I'm sure it is worth it anyway), we can't find any way to play together.

So first question: Is it possible for my friend to play with me in any way without either of us having to upgrade or downgrade? 
Second question: If not, is it possible in the future this may be a feature added to the game (or maybe considering the price of the game make it you get all 4 parts of the game?)