You can make one trip for it.
As long it’s on the same day, you only use one travel day.

How many trips or journeys or trains it is, is not important.

Interrail counts travel days. And on any travel day, you can use as many trains as you like.

If you want to book a reservation for a TGV to Spain, do note that it’s not cheap (Paris - Barcelona 34€ in 2nd class) and that availability is limited. The best is to book it at a ticket office, e.g. in France.

Thank you for your answers !! 

Was in FR a week ago or so-and on many streets there were big advertisements that there are now loads of cheap tickets to visit Barcelona by train (normal tickets, no pass needed), FROM 49€. That is much less as 1 passday+surcharge. No idea how easy these wpuld be to book though

The cheap way to reach Barca-it has been said here so many times; is to get off TGV at perpignan (latest) then switch to local=free TER trains to Port Bou and then again spanish rodalies Catalunya Express along the real costa to BCN-takes longer, saves a bundle