This would be a “mode” or “variant”, like the splitter crossover variant, but on the mixer. That is, at the mix point, it would discard everything above frequency X on channel A, and discard everything below on channel B, then mix what remains to produce the full-range signal.
For example say I like the high-end of amp A, but the low end of amp B. This would give me the best of both worlds, and would open up a lot of tone-shaping options.
It can’t be done with a crossover splitter, since effects applied after the split tend to “escape” the crossover bands. That is, a distortion pedal—even though the incoming signal only contains 800Hz and below—will add coloration above 800Hz to the outgoing signal. The frequency-selection needs to happen after all effects are applied, i.e. at the mixer.