Um....ok, but why "dissimilar"? I'm curious about two different
batteries.

One is an old 1000 CCA deep cycle for trolling and the other is a new 420 CCA
starting battery. Switching them to the both position will cause the good
battery to be discharged by the weaker battery. The way it was described to me
by a battery expert was "If you pour good milk into bad milk you have a bunch
of bad milk".