I'm not experienced in Multi BLD, but I have tried. Memorizing 2 cubes (3x3x3) just by plain letters works fine. I think that 3 cubes will be OK also (For me). Maybe 4?
But...
There is no single "most effecient" way. There are different ways which suit different users/solvers/cubers. You have to find your "most effecient" way. Just get the ideas of others.