hmm, well i myself i never actually used two different sizes of flexwatt on the same thermostat but i would have thought it would work. maybe call rich and talk to him about it--he should have definitive answers.

and rather than buying another 10 gallon tank i might just get another strip of 3 inch flexwatt to use on the 20 gallon. your bp will outgrow the 10 gallon, so why buy another if you don't have to?

EDIT: OR buy another strip of the 11 inch and use that on the 10 gallon you have now, but just don't put all of it under the tank. that way when you upgrade to a 20 gallon you don't have to purchase another strip of 11 inch. (this is assuming that you need the 11 inch to properly warm the 20 gallon tank.)