Hobie1 Hi
I asked once a ticket inspector of the SNCF, it is better to buy two separate tickets. You have sometimes discounts which are valid only in the selected train or not valid on the peak times.
You can also buy these tickets on sncf-connect.com or in the SNCF Connect app, where you can make changes online until the day before departure if the tickets are refundable. It is not possible with SBB, you have to call the Contact Center. Some discounts are also available only with the SNCF site/app.