I just noticed your post and really like that bike.
I haven't seen one like that.
Here's the one that I had found that works for these boys too:
It's a tandem that has 3 wheels and the kids' pedals can be disengaged so that they can rest their legs.
For the boys with weaker trunk muscles, the high seat back and seat belts are useful too.
Anyways, both bikes are great.
Thanks for posting it,