1) Bursting is designed to use 100% of the radio, so this is why you cannot send other messages while it is bursting. The only way to do what you want would be to design your bursts to stop at known times / intervals so that you have windows for other communication.
2) There is only a single radio on an ANT device. Multiple channels are created as a higher level sharing of this single radio. Bursting is designed to use the entire resources of the radio, therefore while one channel is bursting, other channels cannot send or receive. So I would say that no, you cannot reliably receive two bursts on two channels at the same time. Burst was really designed for a 1:1 application.
In general, it seems like you are trying to use bursting in a way it was not intended. Have you considered using a very high message period on a normal broadcast channel instead?