I'm internetting from my phone, so the link is appreciated. The XLS doesn't have a Y mode, only a crossover mode where you must pick a crossover point. So with that power amp, you can't go one signal in and two signals out; you'd need a Y splitter to get two ins to use two outs.
As for other power amps, I'm guessing it could vary by brand and model, so I'd say to RTFM for each amp being considered.
Also not sure if some power amps would allow for odd impedance matches, like an 8 ohm and a 4 ohm cab. However I'd assume as long as your amp could handle a combined impedance number low enough it wouldn't care. I believe an 8 and a 4 gets you an odd combined number of ohms (don't know that math). Of course a stereo amp might be different in an non-bridged mode, but I'm now out of my depth.