Double SxS rifles were a natural progression from double SxS shotguns or smooth bores which were almost without exception, always front trigger right barrel first, rear trigger left barrel second. SxS shotguns with fixed chokes are most always setup with more open choke right barrel, tighter choke left barrel.

Of course now with invector type chokes and trigger selectors the shooter can choose the sequence of choke and trigger/barrel he wishes to use.

Never heard of the left barrel/rear trigger being the normal use of a SxS shotgun or rifle, either chokes for the former or regulation for the latter are setup for right barrel front trigger first.