you can cut it why its in tack, is it as easy as single panels no.
you have to strip the silicone to the point you want to cut then mark the glass (hard part as ur marking in mid air..) then snap it.
no you can not do it if you dont want to cut the silicone to the point. for example tommy has a 10ft tank and he wants to cut it to 5ft he would have to remove 5ft of the silicone in order to snap the glass.
all in all from the look on my bosses face the few times he has done this its most likely easier/safer to just strip the tank cut it and rebuild.