Booking reservations for six - ''reserve max 5''. Not able to buy second reservation when splitted.

I am having some issues booking for six. Paris-Mulhouse Ville, 10.20 on the 28th of July.

the system doesn’t aloud me to book for six, and I am traveling with children so it’s important we travel at least in the same coupè.

And if I try to split the reservation in two (it says it’s only possible to reserve max for 5, unselect some travelers) he doesn’t aloud me to buy the second reservations ( it seem you have already a reservation for this trip) pls help