Never put 6V6s in an amp that wants to see 6L6s. The caveat on that statement is that I have heard secondhand report of people safely replacing 6L6s with - and ONLY with - new stock JJ 6V6s and a rebias. That's because, construction-wise, the JJ 6V6 is in a class by itself, robustness-wise.
However, in fact, it is perfectly acceptable to pull half the power tubes in a Twin or Twin Reverb. You just have to take care to do it right in two ways:
(1) pull them in the correct pair (in a Twin, that's the two middle tubes -or- the two outside tubes). This is because the tubes function in a push-pull paired capacity. A rebias shouldn't be necessary for this (amp tech John Phillips explains why below) but it never hurts to at least check your bias when pulling or changing any power tubes in a Class AB amp.
The only reason it [a rebias] would be [necessary in these situations] is on an amp with a lot of sag in the power supply (tube rectified), where the B+ voltage may rise slightly with two tubes not drawing their current - and even then probably not - but the Twin is SS rectified and has a pretty stiff power supply, so it should make no difference.
(2) accomodate the change in ohmage through a speaker change (what follows are John Phillips words):
When you pull two tubes you get an amp output impedance of 8 ohms - double the normal impedance, not half. To keep things in match, you'd have to double the speaker impedance too, to 8 ohms, but you can't with the same speakers. So, another way would be to set the amp to 2 ohms, if it had such a setting, still with the 4-ohm speakers. Halving the amp impedance is like doubling the speaker impedance.
Now... since you can't change the amp's output impedance (assuming it is a 100W Twin), you have an 8-ohm amp connected to 4-ohm speakers. This is a bit hard on the tubes, although it won't do any harm to the amp [Jon's note: this is because Fender amps generally are engineered to be able to handle this level of mismatch safely]. So instead, if you reconnect the speakers in series (should be easy, assuming the original push-connectors are still there), you have a 16-ohm load on an 8-ohm amp. Equally mismatched, but less bad for the tubes. It won't cause any other problems either, since the power developed is lower and the flyback voltages won't be any higher than for normal operation.
Alternatively... if you're still concerned with volume, just disconnect one speaker as well! Instant 8-ohm load.
Last thing I'll say now is that someone - like jonarobb - may not personally prefer
the tone of a Twin running on one pair of tubes but never confuse this with the ability to do so safely.
The content of this post addresses the latter. Thousands of guitarists have, and do, run their Twins safely this way and prefer the attendant volume and tone changes (though the actual volume change may be less than you'd anticipate!)