If I had a double on a 2x2x4 enclosure -- would a 6% still be efficient? If I wanted to do one uvb and one plant?A double will reduce output by 20% per bulb So keep that in mind.
So a 6% in a single is putting out more UVB than a 12% in a quad.
A 12% in a double is putting out 50% more UVB than a 12% in a quad.
Id say a 12% is a good choice, and just lower the basking stick to around the 12" level to compensate for the 50% higher output vs 6-7" away from a quad with the same 12%, or a 6-7" away from a single with a 6%.