Is that a bad way to do it?
Well for one, idiots like to call them "free-boards" because you don't have to purchase a board. But you ruin a perfectly good snowboard, so its not any cheaper since you need to replace the snowboard you drilled holes into.
Second, snowboards aren't built to bear a persons weight when the board is suspended above ground from 2 points. They're built to evenly spread a persons weight across snow.
They're not nessicarily "unsafe", but they're definitly not
as safe as actuall long boards.