Nice looking tandem. I couldn't say the exact year but I'd guess it to be maybe late 70's. It would have been aimed at a similar market segment as our '82 Peugeot. It shares some of the same components and configuration.

You asked about several technical things.

For the seatpost, measure it with a good caliper. Places like Harris Cyclery typically have or can order a seatpost of any diameter. The thing to be careful of though is that the stoker's stem must fit it too. If the original seatpost was fit into the captain's ST through a collar (like on a UO-8) or if the original seatpost was dual-diameter, then stoker's stem may want a different diameter from what the frame wants.

Your cranks, at least on the timing chain side, are TA (or a lookalike) and will require a TA crank puller. Pompiere was half-right about it being French; it is French of course, but you might be mislead into getting a Stronglight puller which is different still. TA pullers should be available if you look hard enough. Not necessarily cheap though. Unless it was build after the French started migrating to more universal English standards, any decent French bearing cups should work for the BB's. Remember, if French both sides use right-hand threads, and the right side must be torqued in quite tight or they will come loose.

Those brakes are almost like ours, Mafac cantilever, Atom drum. Our bike has canti's on the rear as well as the front. It was rigged by the factory with the dual cable on the right hand, the drum on the left. I kept it that when I built the bike up from a bare frame because the cable housing braze-ons worked better that way. I would have preferred it with canti's on the left because all my other bikes have front on the left (the traditional European way). I usually just operate both levers anyway. The drum does add significant stopping power, though its original purpose was to let you control speed on a long descent without overheating the rim. It doesn't require much maintenance as long as the linings are in good shape but you'll need a "special" tool to service the bearings if it is like ours. The left-side bearing cup requires a 15mm cone wrench but you can't get one in there because it is down in the drum well. I took a 15mm cone wrench with longish handle section, and bent a curve into it so I could dip it down into the well. Bending it was hard! The first time you pull the left side apart you'll see what I mean. It isn't tricky though.

Tires and pressure? We just use Pasela TourGuard, 32mm, pumped up to between 95 and 100psi (their nominal max is 95). You are carrying more weight than a solo bike so more pressure would seem advisable. I replaced the rear tire at about 3500 miles, but probably didn't have too. We've ridden upwards of 6000 miles over the last 4 years and never had a flat, but did pick up one sliver of glass in the rear last year which gave us a slow hard-to-find leak.

As for riding, ah, that's a whole 'nother kettle of fish. Several rules must be followed. The Captain is the Boss, period. Job 1 is to keep the bike safe. Job 2 is to keep the Stoker happy. The Captain does all the steering, braking, turning, gear selection, etc. The Stoker's job is to pedal, enjoy the scenery, and the tell the Captain what he/she did wrong. In the latter case, the Stoker is always right. The Stoker can't see through the Captain but the freedom to look aside usually means he/she sees so much that the Captain can't see because he/she is focused on the road surface 20yds ahead. If the Captain wears a helmet-mounted mirror the Stoker must never try to peer over the Captain's left shoulder to see what is in front.

The Captain can't see or hear the gears. If you have a triple crank and the Captain asks what chainring you are on, the Stoker should answer Big, Middle, or Granny. "Little" sounds too much like "Middle". Most shifts don't have to be called out, but shifting to the granny ring does because the higher chain tension can make that shift problematic. The Captain must avoid bumps (you're carrying a lot of weight, remember) and call out those that cannot be missed.

Develop a procedure for starting and stopping. Decide which pedal will be down by default for most situations, even coasting. When you are coasting, especially over a series of bumps, the Captain should call out when to start pedaling again. The reason is the Stoker may have lifted off the saddle and will be surprised when the pedals start to move.

Enjoy the ride!