Are different stereo amp models in same system ok?

"Must I use the same amp model for a 2nd amp?"

Yes, unless you are going to use an outboard crossover,
which would have volume (level) controls to match the output
of two different amps.

http://www.audiokarma.org/forums/showthread.php?t=458761

This is one of those topics that has been discussed many times on the forum with different opinions. IMHO, here is one of the best answers from a well respected amplifier designer,

http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1234896660&openfrom&6&4#6

When bi-amping with identical amps, the vertical configuration is best, however another thought comes to mind. The woofers require and use much more power than the midrange/tweeter. If you consider just one speaker, what makes better common sense, to use a 150 watt channel to the woofer with an additional 150 watt channel to the midrange/tweeter, or use a single 300 watt channel to the entire speaker. Obviously, the 300 watt allows a greater benefit for the woofer.

Also, when bi-amping with an outboard crossover, I have read that it is best to bypass the crossover in the speaker, which seems to make common sense.

Then there is the additional expense, cables for sure, and crossover for active bi-amping. I would rather just use the available funds for a quality stereo amp or a pair of quality monos.

Would you care to share the speakers you are using, and reason for upgrading the amp?

no.....as a matter of fact, one good amp is worth 2 not so good ones.

You needn't use the same model. In fact, many people use ss for the bass, tube for the mids and highs. I've been doing that for years to great effect. SS amp has 3 times the output, but it has volume pots, which get turned back to about 40%.

Well, since the 1954 Brown vs. Bd. Of Ed. Supreme Court decision it is.