Lets hold on a second here. What do you mean "ampwise" it's maxed out. Have you done a real load calculation or are you just summing up the breakers in that panel?

Even if you were "maxed out" what good would this "switch" do?

Large double throw switches are availble, but the price will probably be scary to you. The cutoff is about 30A for the kind that mounts in a regular box versus the ones that come in their own NEMA enclosure (and the accompanying price jump).

If you're really determined, your cheapest answer would be a 40A DPDT Intermatic timer (whether you chose to use the "timer" part of it or not).

Why would you want to do this?

"There is room physical for additional breakers in the Panel B, but amp wise it is maxed out"

Based on what?

The sum of the breaker ratings in the panel?

Only an actual load calculation can determine if the panel is "maxed out," not adding up ratings of breakers.

the max load calculation is 180 amps in panel B, however after I have turned off one of the hot water heaters (only my wife & I) and because we have both a geothermal boiler for in floor radiant heat and a forced air geothermal unit I disconnect one of the units using the manual breaker mounted on the wall next to the geothermal units. The main reason I want to do this is cost conservation. interuptable power is much less expensive, but on the rare occasion we have a full house I'd want be able to put the stove top and oven back onto panel A.

A transfer switch like used for generator installtions sould do the job for you.