I've been reading about Sir Mackenzie Bowell, one of the less notable PMs that served in Sir John A's wake. It makes note of the fact that he, along with another PM, were members of the Senate as Prime Minister.
My question is: Who would lead in the House of Commons in this case?
As I've established before here, electoral history is a great love of mine, so please excuse the flair!
Within the Westminster parliamentary system, it is not necessary for the leader of the government to also be the majority leader of the lower house, though it is certainly more convenient (and today, surely unthinkable for anything else to be the case, at least in western liberal democracies). In the absence of the Prime Minister serving in the House of Commons, a Leader of the Government in the House was chosen in Bowell's stead; three Conservative MPs fulfilled this function during Bowell's premiership. The last, Charles Tupper, succeeded Bowell as Prime Minister.
Source: Robert Marleau and Camille Montpetit, House of Commons Procedure and Practice (2000). Digitised here; see footnotes 3 and 4 to the table of party leaders.