I have a similar vintage Gitane tandem. If you have a Stronglight 99 tandem crank installed, there are compatible Sakae Ringyo middle rings with a ramp to help prevent this problem. I may have a couple in my "big box o' chainrings" if you want to try one. Gitane also used TA cranks on their tandems at times, in which case my chainrings wouldn't fit.
If you do replace the chain, don't get one of the modern super-narrow 9 or 10 speed chains as these will exacerbate your problem. I've used SRAM 7-speed chains successfully here.
Regarding the long chain line -- you can mount the drive crank on the rear bottom bracket to shorten the chain run so that a single road chain will suffice. I found the long chain had a tendency to bounce off the tensioner when the cranks were mounted on the captain's bottom bracket. The downside is that the derailleur cables are much longer, and the original shift levers on my tandem (low end Simplex) did not work too well with the longer cables. I soon replaced them with Simplex "retrofriction" levers which work much better.