One of our Islabikes was s/h, used by both my kids and now sold on to another family (for almost the same as we paid for it). Islabike currently being ridden by my oldest is also s/h – bought from somebody I know, had been used by two of his kids and I’m sure it will be fine for younger one when it gets passed on. s/h was actually an advantage for me at this point, as of course when we got that the younger one got handed down a bike and they’re old enough to be aware of such things – so they both have s/h bikes.
I’m not sure why you think there would be a problem buying a s/h quality bike for use by two kids – they tend to grow out of them rather than wear them out.
Oh and we did the bike for Christmas thing once, but if you’re buying quality bikes and handing them down it’s not a good habit to get into – bikes are now bought when they need them, and not for presents.