Why isn't the application of "carb heat" in initial piston multi-engine failure emergency procedures?
Partly because carb heat is ineffective once the engine is no longer making power, but mostly because the initial engine out procedure isn't intended as remedial action to get the engine running again; the drill is to make sure the airplane is configured to be able to climb (somewhat) in the moments after the failure, so the priority is everything balls to the wall, gear and flaps, identify, confirm, feather with the aim of establishing a climb foremost.  

In any case, if you applied carb heat to both engines as part of the drill, the dead engine would be getting probably insufficient heat anyway, and the live engine will enjoy a few percent power reduction from the application of heat.  On a piston twin like a Seminole or Duchess at gross on a hot day, that alone may be the difference between barely climbing or descending, so that the remaining engine takes you directly to the scene of the crash as the old joke goes.