Crossover - hard to find for biamping

I've had good luck with the DBX crossovers, just be sure you get the one with the audio connector not the 1/4 inch jack.

I’m bi-amping with tube monoblocks into the midbass and tweeters using the passive crossovers, then running a solid state amp into the woofers. I’ve split the preamp signal so that one leg feeds an active low pass crossover to the woofer amp. It’s a very inexpensive Pyle PLXR2B for automotive use, but using a suitable 12V wall wart it works remarkably well. It has low pass options of 50, 63, 80, or 100 hz, and a separate gain.  Not much to lose at $30.

You only need to adjust the levels if your amps have different gain.  There's a convention but not a requirement for that.

If not, any passive volume attenuator will work. Put it before the loudest amp. :)

Wait

You shouldn't need a crossover if you are using the existing built in crossovers just volume controls.  It might get tricky if the amps have a different gain profile as you adjust for one global  volume level and then if you increase or decrease the volume the bass amp and the mid tweeter amp are no longer in sync.  It may not be much of an issue as you can always just increase one of the amps volume level to compensate.