We've taken our tandem on a lot of Swiss trains, and a few French ones. So, I can confirm that there is not a universal policy banning tandems from all French trains. However, whether it is feasible or not really depends on the specific train, and there is a lot of variation in that. Our tandem has couplers, and more often than not we split off the front third (easy to do if the couplers are in front of the captain's seat tube) to hang each part vertically from the hooks that are only high enough for single bikes. On some trains, there is an area for horizontal bike storage, and then the tandem normally fits without any disassembly, on some lines we know that this will be the case, but a lot of the time we don't know this until the train arrives. Some trains even have a goods wagon or guard's section where bikes should go, separate from the passenger compartments. These normally have loads of room, and so are not a problem with a complete tandem. Not knowing what to expect when the train arrives, and having a tandem without couplers could be a problem.
In France, you will probably have a choice between taking a TGV (fast train) or a standard regional train. Most TGVs have a bicycle compartment (but not all). The TGVs that I've used have all had hooks on the ceiling, with enough hanging space for single bikes, there may be enough space to put a complete tandem, or two, next to the wall in the small bicycle compartment, but I don't know whether they would allow this. You have to pay and reseverve the bicycle space on the TGVs, but not on the regional trains. The TGV passenger tickets will also cost more than the tickets for the regular trains, but some times you can get a special price for the TGV online, particularly if you book a couple of months ahead of time.
My best advice would be to try to contact the TGV (who operate the fast trains) and SNCF (who operate the regional trains) and see if they can offer you any info. However, don't be surprised if you have a hard time getting anything out of them. I also just found a useful link, with links to other info on taking bikes on trains in Europe, some of them specific to France, see here
, in particular, there is a report from someone
who successfully, but with some hassle, took a complete tandem on some regional trains in France.
Best of luck!