France regards its nuclear mission as sacrosanct and a national obligation. They will be very unlikely to 'share' anyone else's tankers to support this mission.
I think there is growing realisation on both sides of "La Manche" that we are going to have to share a number of major long-term strategic procurements or lose them completely. The French army and Bundeswehr already have a joint brigade, which has set a successful precedent. The French navy have occasionally expressed a (mostly non-reciprocated) desire for a joint naval carrier group with the RN, and even whispers about a joint nuclear submarine deterrent.
A joint FSTA programme is not that impossible by comparison.