Replacing two 16.5V transformers with one
Focus on commercial, off-the-shelf

The idea of a single transformer is fine, just get large enough as you plan.  However I would dissuade you from acquiring one out of an electronics supply, because they are not UL listed as mains electrical equipment. 

Appropriate equipment these days will typically screw into a junction box knockout, or mount on the lid of a junction box.  It is sold either at electrical supply houses or heating/cooling supply.  Amazon is a fine source as long as you watch out for "sold by Joe Blow and ships from Amazon" types, make sure they are selling reputable UL-listed items and not the usual Chinese garbage off Alibaba. With mains power, don't fool around.