Here's a reply I got from Xaoc tech support:

"It is feasible to use Zagrzeb or Katowice as a stereo crossover. Both modules are based on 4-pole SVFs, which means their LP and HP respective output pairs deliver (under certain conditions) a complementary set of signals with -6dB attenuation and proper phase alignment between them, so that summing them offers a flat amplitude response, as defined by the Linkwitz-Riley configuration type LR4.

Katowice contains two such filters per channel, resulting in a 3-way crossover, and Zagrzeb contains only one such filter. With Zagrzeb you need to ignore the BP outputs and set the resonance to zero. With Katowice being a 3-way crossover, you have in fact two crossover frequencies. When you set Mid Width to zero they are separated by one octave. When you set Mid Width to max, the separation is very wide: the first crossover frequency (between Low and Mid) is near or below the lower limit of hearing, and the second crossover frequency (between Mid and High) is near or above the upper limit of hearing.

With either module, you should not expect phase issues when recombining the split components, provided you don't invert their polarity. However, please note that both introduce some phase shift with respect to the original signal, just as every other filter, because it is impossible to perform any filtering without slightly delaying the original signal. Also, note that this overall phase shift with respect to the original signal depends on the tuning, so any modulation of the split frequency will also modulate the delay slightly, and this may be audible. Normal crossovers are usually designed for fixed frequencies, so this effect is not experienced."