Question on using bi-amp...

i do not know internally bat units but i assume that these outputs shunt each other and cannot work together at the same time or even the ground loop problem may occur when you will try to connect both at the same time. the regular way is to acquire either y-connector or active crossover.
for turning ON and OFF you should arrange an A/B switch but why would you want to use one of the amps if you're ready to bi-amp or how the hell many A/B compare tests you should run to find out the best point??? if you unswitch unused one you will have to connect jumpers to the binding posts(do you have bi-wired speakers?) so you will not escape connecting and re-connecting buzz anyway...
i have an ear-speaker adapter and speakers and i mounted toggle switches onto each of my monoblocks and thus i can re-direct my amps to drive different speakers.

connecting two amps to one same terminal will result in frying one of the amps or even both depending on which one is on. In my VTL MB100 the output impedance is 0.3Ohm that means that if I would connect it along with the other amp(some of them have even lower output impedance) the resulting load on each amp will be almost zero which certainly will result either blowing of an output fuse or even output transformer. you should definitely use an A/B switch and please do not call it bi-amping. now having a tube amp you must realize that if you leave it on without loading unswitching A/B switch you will fry the transformer so you should arrange a dummy load resistor while one of the amps are not operating but stays on(would you turn-off tube amp every time??)
as far as I know that your bat can be re-arranged as mono and you can buy the other same bat. many people have done this so far and have a great improvement in performance of 120W per each side.

if i had bat i would definitely contact viktor that is always helps either thorugh e-mail or phone. please make a note that bat units is very costly $$$ to fix! you can also buy a pair of bat monos(usually sells cheaper per each than one stereo amp) and than sell yours.