This is a good question. First, I use technaflora which has one recipe throughout until the flush, so for me there isn't that issue. Other brands have their own formulas but if you look closely it is mostly an even recipe of macronutrients that just rises around the 4 th week and falls back 2 weeks later. So you could just continue flat and not rise and that would be just fine by the plant, or you could average the two (especially on a light feeding schedule) and use that number.

The other reason though not to mix strains is because one strain may drink certain nutes out at a different pace than the other leaving one more deficient and how would you do your flush at the end?

Those are your challenges!