#1
I have a B52 head that I got from a friend and he had 6L6 groove tube power tubes in it. Two of them blew out and I'm wondering if I can replace them without needing to set the bias of the amp. Could I use JJ 6L6GC tubes on just those two while leaving the working groove tubes in and not need set the bias, or do I have to use the same exact tubes that are already in it? I'm not confident enough to bias it myself so I want to see if I can just put new tubes in
#2
It's best practice to bias each time you change tubes but IIRC the B52 is biased pretty cold so you can probably get away without it.
You should not mix new and old tubes. If two of them blew (I assume you're just looking at them to tell, not a great indicator) then the other two may be on the way out. It's also possible that the two that "blew" are fine and the heaters are out or something is wrong with the amp, so be aware that there's a chance the amp will need to go to a tech.

Anyway, you can run the amp with only two power tubes if you're trying to save money. Just put the output selector on the next lowest impedance setting - for instance, if you have an 8 ohm cab, change the selector from 8 to 4. Then put the new pair of power tubes in the two outside sockets. This will run the amp on half power, which should still be plenty loud. Better to do that than run two old and two new tubes.