I do have a 70's tandem, for what you are asking about I think is really more the materials made from than just the time period.

My 70's Rodriguez tandem is solid as a rock simple front triangle and unsupported diamond single seat stays Mafac canti brakes, it has an ovalized tube on the bottom, it also has a 1 1/8" steertube I think, the joints (trust me they are not pretty) are crude. This has French downtube shifter braze on, and uses a claw derailleur. I am assuming this was a prototype givin the laquer 'set' brand decals.

Gitane made some super flexi all round tube unsupported diamond frames, they also made some that hade 1/2" tubing that ran length wise. Those may have been stiffer. I really don't know much about 70's tandems.

The Jack Taylor I heard were superb. Rodriguez was know for good tandems back then, or at least after mine was constructed, lol.

I recently picked up a late-mid 80's all 531 with the ovalized bottom tube with double seatube stays. I have not had the time to work that through all the maintenance paces to test try.

At one time I, with my brother, had the '74 twin Schwinn 3 speed kick-back. We never got that one going. I ended up donating that to local.