Shown is my bicycle on the left and my wife's bicycle on the right. If I were pairing two bicycles, I would pick two that are more similar, and would probably not use one with drop handlebars.
Step 1: Keep the bikes next to one another
If two bikes are to be paired like this, the bracing cannot interfere with the normal movement of the pedals, the feet, and the legs. This diagram assumes the rider will ride from the bike on the left. The bracing could be flipped if the rider were to be on the right side bike.
I would remove the crank and the chain on the bike not ridden. Not only does this lighten the bike, but it also offers more places to connect things.