I have never known what the actual rule is for this. It doesn't really matter unless you actually want to tell someone what it is.

The way I've been figuring it is to find the AR for each wing and average them together.

The other way I've done in the past is to take the average span and divide by the distance from the forward most leading edge to rearward most trailing edge. I'm fairly certain that's wrong though.

What's the proper way?