Towing anything by paddle will be a job.  Anything less streamlined than a canoe will be a PITA.  But if you insist, here's how to connect the towed object.  (This comes from having towed a loaded solo canoe behind a tandem some years ago.)

This arrangement will prevent the towed craft from slewing sideways badly and eliminate the bumping involved in rope towing.

Don't use a rope.  Use a curved stick about 6 feet long; it should have at least 2-3 inches of curvature.  The stick needs to be arched to clear the deck plates of the two canoes.  If you aren't using canoes, you might be able to use a straight stick.  Attach one end to the stern of the tow craft and the other end to the bow of the towed craft.  If the stick is curved, the arch, should be upward, and the method of attachment should keep it that way.

A much better approach is to use the proper craft so you don't have to tow.  If you're concerned that a canoe won't be stable enough, do the following:

1.  Learn to paddle a canoe.
2.  Get a canoe with a shallow-arch hull and enough capacity to carry what you want.