So maybe the beach rental clunker may be the right thing to get a feel for the coordination of "twogether" riding.
I'm sensing from some of the posted comments that the basics of balance and coordinated pedaling are the steep part of the learning curve and take the longest to master --- and these don't involve gearing or lightweight frames.
Is it fair to say that a quality bike makes being proficient more fun; but, it doesn't make you proficient faster?