It's entirely possible. Just build the amp for 6l6's. Some people think that over reated iron is a detriment to tone. I don't. But everyone is entitled to their opinion.
Three things that need to be considered and designed into the amp are:
1) Plate voltage should be reasonable for both tubes. For an amp that will run 6l6's or 6v6's that basically means you don't want too much voltage for the 6v6's.
2) The OT primary impedance is a touchy subject for some. I'm not in that camp. I would probably use a 6.6k primary impedance and call it good. Others would probably use an 8k or 4k primary and use the secondary impedance taps to reflect 8k primary on the 6v6's and 4k on the 6l6's. Still others might use base the primary impedance on plate voltage, tube dissapation and bias condition. Way overkill for a guitar amp IMHO.
3) The bias condition for either tube type must be accomodated.
There's really nothing more to it than building a 6l6 amp that doesn't have to high Vp for 6v6's, use a primary impedance that's good for both tube types OR have the primary impedance selectable for both tube types and be sure both tube types can be biased properly.