Best crossover frequency when bi-amp

How do you determine the best crossover frequency to use when bi-amping speakers (no specs available)?
Your question seems to imply that you are not using your speakers internal crossover, i.e. passive bi-amping, and wish to add an active external crossover. This adds considerable perplexity to a solution that you might wish to be simple. Not to mention the not necessarily easy task of disconnecting the existing crossover network (if it exists) for your loudspeakers.

The crossover points for a speaker system depend on the type of drivers (planar, ribbon, cone, etc.), the drivers specifications, the size of the enclosures, the type of the enclosures (vented, transmission line, etc.) ... not to mention the construction materials and methods of the enclosures themselves. All this STUFF (and more) is what those very clever people that designed your speaker system took into account when they specified the crossover(s) frequency and slopes. You could be playing games with your crossover for quite some time just trying to find a tolerable listening setting before you ever get to the point of just tweaking for the hell of it. And in the interim, the sound may be abhorent.

Silly question, but can’t you contact the loudspeaker manufacturer to obtain crossover frequencies and slopes? Or failing that, perhaps you could post specifically the need for your speakers crossover specs here, for those folks that might own and know them, or even fall into the group of clever individuals that actually do design them? I would take one of these much simpler routes before I ventured into the speaker design business.

My opinion, for what it’s worth.
Thanks for your valuable input, Massvm. I'm toying with a pair of very old JBL horn speakers. Don't even have the model number.
Ryllau, JBL maintains a pretty good database of vintage equipment at their website. I wouldn't discount doing some wandering about to see if you find something there. You may even try contacting them with a digital photo attached. Worth a shot.
Thanks Massvm. Will try JBL site.