When you biwire, you do not send the full amplifier output to the tweeter, do you? Of course not, you’d blow the bugger out. It still goes through the speaker’s x-over. It’s easy to verify with a DMM - just check for continuity. Spendors used to be wired that way, IIRC, and it always puzzled me.
I agree that you should check for continuity before bi-amping, Check hot to hot and ground to ground. I’ve seen all 3 cases:
- Infinite impedance
- Both internally shorted (0 Ohms)
- Ground only shorted
In the case of 1, bi-amping is fully safe.
In the case of 2, no bi-amping at all is safe
In the case of 3, only unbalanced amps with 1 hot output are safe.
In all cases listed the internal crossover remains in place. It's really a manufacturing choice whether or not the filters are joined internally.
Want to point out that there’s a number of A’goners who are successfully passively bi-amping. 1 and 3 seem to be the most common situation.