You can switch to one handed for spares and vice versa as long as the ball is in the same dominant hand. USBC release a statement about that on the bowl.com website either last year or the year before. That is for adults, I believe in youth leagues/tournaments they have to use one or the other.
Averages are still maintained and all USBC sanctioned averages have to be maintained using either one hand or the other and no combination of both.
If youths are not allowed to switch, it might discourage them from using different styles or cause them to have lower scores. Then they may never become pros that switch between styles. What good is that?
I guess the popularity of the two-handed style is still fairly new and the rules haven't quite been ironed out yet.
(I freaked momentarily when someone joked about the helicopter technique not being allowed in league or tournament. haha I had a stronger reaction to the two-handed issue. I just hate the thought of an organisation putting unreasonable limits of individuals.)