Depends on several factors. Frequency range, install locations, specific drivers we're discussing, etc.
Are you doing a kickpanel install or strictly in the doors? What frequency range will the driver(s) be covering?
Would I run a single 8 in the kicks instead of a pair of 6.5's in the door if it were running into the midrange? Probably. Kicks are a better install location for PLD and aiming. Dual 6.5" mids are only going to net you about a 2db increase in output over a single 8" (all else equal), which is an "audible" difference but not a large audible difference especially considering you're doing good for yourself if you can get your response to within +/- 3db. Plus IMO it's "easier" to build kicks than rebuild a door panel.
And depending on the frequency range you're running you might run into problems with cancellation between the two drivers in the midrange frequencies (probably not as much of a problem if you're running horns since the lowpass will still be in the lower midrange frequencies). But in a 2-way with a standard tweeter crossed at 2.5khz+ this would be more of an issue.
But that's not to say a pair of properly done 6.5" drivers can't sound good......as I'm sure Horseman will attest. It just comes down to particular circumstances. Heck, I have a quad 6.5's sitting around I was debating on running in my doors as midbass just for shits and giggles //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif But I might stick with an 8" for midbass. Still undecided (Haha! Which I guess in the end means we're ultimately in the same boat //content.invisioncic.com/y282845/emoticons/tongue.gif.6130eb82179565f6db8d26d6001dcd24.gif )