So it was as uncommon then as today?
well it was not exactly uncommon for a fairly long time... You also see Cartier and Turlier double signed pieces. Seeing a stainless steel double signed is really rare, and a cartier double signed in steel would be very very rare if one exists. I am pretty sure I have seen cold Tiffany signed speedmasters from the early 70's, and maybe some more dressy ones from the 80's.
My point was it is not as cut and dried as you made it out to be.