Horizontal vs. Vertical amps: If you use one amp per side you have better channel isolation and any power modulation is "all together" with the signal. You can also place the amp physically close to the speaker and use very short cables from amp to speaker. If you use one amp to power bass channels and one to power highs, there would be less modulation of highs by lows (and vice versa) through the power supply (etc.) In an active system you can also use a smaller amp for the highs. I have done it both ways and am not sure there's a clear winner, but sonically most (including me) prefer to use a big honkin' amp on the bass and smaller, "smoother" amp on the highs. The most popular combo (in the audiophile world) that comes to mind is a SS or hybrid bass amp and tube mid/upper amp. That is what I used to do with my Maggies.
It is possible the load is "easier" on the respective amp channels due to separating the low and mid/high impedance variations. That would be quite speaker- and amplifier-dependent, natch.