I really like your concept and I have been thinking about it for a while. The problem is finding 2 attachment points on the backpack to hook a shoulder strap to and use it as a shoulder or cross-shoulder bag when using the other backpack in "backpack mode"
How about carrying one of the backpacks on its side - use the waist strap gatekeeper loop as one shoulder strap attachment point and the hand webbing as the other. You can put in those round metal key chain loops on the attachment points to make it easy to clip in the shoulder strap (there are other options too). You can even use the other waist strap gatekeeper loop to put in a stabilizer strap like the QAM version or the separate Ristretto bag one you can buy.
I will try to take a picture of what I am describing when I get a chance.
Hope this helps and I hope this is in line with what you are asking.