I think too is they just have to want to, and neither of them seem to have much of a desire or motivation to try, iyswim. Like I said, DS1 gets more motivated when he sees other kids. The other day I packed them up with their bikes and drove to the park around the corner (even though it's close enough to walk!) because there is a big basketball court there and I figured they could practice. Well, neither of them wanted to.
But then some kids came along after a bit and were riding their bikes around the park pathways (one about the same age as DS1 with a similar bike) and all of a sudden he wanted to try. He was doing well, but again once he loses momentum and stops he can't quite get going again, especially if it is on a hill or there is a bump. So I think we will have to just keep trying. DS2 just gets off his and walks.
But if I could take him out with just the two of us and push him along, he might enjoy it.
I doubt it's like they will never learn to ride bikes, they just don't seem to care much about it atm! I suspect it would be the same whether it was a balance bike, tricycle, or bike with training wheels.