On the one leaf one do you see a small green knob at the base of that leaf?

If so it's what one would call a blind seedling and the chances of seeing another leaf coming out, and growth of the plant are low.

But since it's one of only two seedlings you have of that variety, I'd just let it go for a while.

An alternative, which to me is more attractive is to let the good plant go a bit and then take a couple of sucker cuttings to root for new plants.

Carolyn

Agree. It happens maybe 1 out of 250+ seeds IME, maybe more, from a damaged or immature seed and they seldom survive. Like Carolyn said I'd let the healthy one develop well and then root cuttings off it.

Dave

Thank you for the advice! I'll let it go a while longer. There is a small knob near the base of the single-leaf plant. So I don't have high hopes. I will take a cutting from the other plant in a couple weeks. Thanks!

I am an optimist ; Let it be.
I would also loosen up the soil around the base (with a toothpick). Chances are that a stem will grow out of that knob.

It makes a nice experiment.