Jack, the Honda's do that automatically.. HOWEVER my answer in the OTHER thead on this subject tells how to figure out if it will work.
I will explain
IF, as in the case of the cheater box I have, the two 30 amp (in my case) or the 30 and 20 amp plugs feed different legs One feeding L-1 only, the other L-2 only, then it will work.. In this box only white and bare/green are common, BlACK (The hot wires) are independent of each other.
I still do not recommend it.. But it should work. Likly NOT the way the O/P thinks it will, but it should work.
If however the black wire is common (Very dangerous) it will not work.
