Strange that you should strongly not recommend it, as it is actually the way a number of companies I know in Australia do it, and it works wonders. They traditionally run a 3 month season and 3 month rehearsal proccess, and all of their SM's are company SM's, not show based (which I suppose makes a big difference here, because it means that is what they are used to).
SM1 attends all rehearsals, does all the "SM duties" and is often just called the RSM (Rehearsal Stage Manager), whilst SM2 (Calling/Production Stage Manager) comes in for maybe 2 rehearsals near the end, the production meetings, and meets the SM the day before tech.
Tech week I call the handover, because RSM and CSM are both working at the same time.
CSM deals SOLELY with technical stuff, and from Tech Week onwards is basically the SM in charge. RSM basically makes sure that the actors are comfortable, and that all the props made it from rehearsal room to stage (so working closely with the ASMs to make sure everything there is ready), then basically sits next the the CSM for tech and makes sure they know everything they need to know.
The system works well because you end up with two highly specialized SM's, and a year long continuous run of shows.(nb: this system also usually involves "ownership" of a room or venue. The three companies I have seen using this method had multiple theatres and rehearsal rooms, so an SM basically "belonged" to a theatre.