If your head is truly bi-amp capable (ie. has two amp stages in a single enclosure), the answer should be yes to both. The primary advantage to having a head with true "bi-amp" capabilities is the ability to use simultaneously 2 different cabs with substantially different impedance/power handling specs covering two different frequency ranges without having to worry about grossly unequal power distribution, impedance mismatching, massive explosions etc.
Keep in mid that there is a substantial difference between a head with true "biamp" capabilities and a head that merely has two (or more) speaker connections and specifies "ratings per side." In the former case, you effectively have two amps, each driving its own cab. In the latter case, you are connecting two cabs in parallel to a single amp, and there you *do* have to worry about matching impedances and possibly frying your equipment. If the amp isn't truly biampable, connecting an 8 ohm and 4 ohm cabinet in parallel is a reason for worry.
If you're at all unsure as to to how your amp is configured, definitely contact the manufacturer or a knowledgeable tech before you try anything.
[Edited by Christopher on 08-09-2000 at 04:59 PM]