A cab does not have to be "specifically designed" for bi-amping. What a bunch of BS. As long as each cab has a strong response in the range you need it, and enough power handling to reproduce that frequency range at the volumes you need it at, and sounds good doing it, it should work just fine in a bi-amp setup.
Now obviously, it's theoretically possible to get better results if everything is designed to work together, but it's not strictly necessary, and there's no "rule" that says that a speaker HAS to be "designed" for biamping (unless you're talking about bi-amping within a single cabinet).
And regarding matching vs non-matching cabs, I think this is one of the biggest myths here at TB. I'm not saying the science behind it is a myth, I'm saying the importance of the science in this application is SEVERELY overstated. As already mentioned, phasing issues happen EVERYWHERE (literally). I think the only way you're likely to hear sound that doesn't have any phase distortion is if you were completely deaf in one ear, sitting in an anechoic chamber, and listening to a pure sine wave from one speaker driver. As soon as you change one of those variables, you're going to have both phase cancellation and reinforcement. And those will change as you play different frequencies and/or move around the room (in relation to the speaker/s).
Using all matching cabs could reduce some of the phase changes before all that other stuff comes into play, which could certainly have a positive effect. OR you could just as easily be undoing phase changes that could even have a positive effect tonally.
People don't understand that MI amplification is a funny thing. It's not necessarily linear/flat/perfect/etc..., in fact, it's usually not any of those. Guitar cabs roll off after about 4-7KHz, and if they didn't it would sound like crap (record a distorted guitar from the line out of the amp and listen back on studio monitors if you don't believe me). The "perfect" tones we all love were all created using imperfect means, which is why trying to recreate them using clinical means is a pointless and futile task.
By all means, if you have one cab that you love the tone of, get another one. Then you will have the same exact tone, just 3db more of it (plus whatever advantage you might gain running a lower impedance load on your amp, which probably isn't much).
BUT, if you're like most of us and find that 1 cabs gives you a little of this, and another cab gives you a little of that, if you like the way they sound together, go for it. The most important part is the sound, the audience isn't going to be listening to phase response charts at the gig or in their car while your CD is playing.
Unfortunately this means the best advice is just to try out a bunch of different options with your current setup and see what works best FOR YOU. 2 15s can be a cool sound, a 15 and 2 10s can be cool sound, 4 10s can be a cool sound, etc... Out of the 4 cabs I have, my favorite setup is the Trace 1x15 and matching 2x10. The 15 has deep, powerful, low end and lower mids, but rolls off pretty fast higher up. The 2x10 is the reverse of that. Neither is perfect on their own, but together they are pretty magical (and that's without a tweeter, but they still get a pretty "full range" sound). I love my vintage Carvin 15 too and it's got a great overall sound and I wouldn't mind having 2 of them. But it lacks the intense low end the Trace has. The 2x10 is kind of thin by itself, but with the deep and warm Trace 15 that becomes an asset rather than a liability.
