If they fit in SNCF's required dimensions, then you don't need to book them:

“When you pack your bicycle in a carrying case, it is treated as carryon luggage. It must be tagged and cannot exceed maximum carryon dimensions: 120 cm high x 90 cm wide.”

See:

https://www.sncf.com/en/passenger-offer/travel-for-everyone/bike-on-board

How do I reserve a space for 2 bicycles on SNCFConnect, since I do not need a ticket simultaneously. We have global passes. 
Christian

You'll need to call SNCF:

https://www.sncf-voyageurs.com/en/customer-service/contact-us/telephone/

Press #85 for English.

We travelled from Nice to Paris on the TGV with 2 assembled bikes last year without Interrail passes ie ordinary tickets. This involved a change at Lyon Part-Dieu. We had to book 3 months ahead to secure the reservations-it was very difficult to make them as there appeared to be only 2 bikes per train and only one day train each day that accepted bikes. For the night train there were more bike spaces but with a bike at night you can only travel in a 6 person couchette which we didn’t want to do. 
maybe look on SNCF Connect to see if there are any bike spaces for your date for non-Interrail pass holders as a first step and if there are phone SNCF? 

Thank you. Well, The thing is we have passes. On SNCFConnect you cannot book bicycles, without booking a passenger ticket at the same time. This has apparently not been foreseen by SNCF app architects, nor by Interrail. 

Hi-yes I realised that you have passes-but if there are no bike spaces left for people buying full tickets there will be none for pass holders either. If there are no TGV bike spaces left for your date you could do the journey in several reservation-free RER trains (some of which might need one of the more plentiful RER bike reservations). There are people on this forum who can advise you on reservation-free routes if you give them your travel date

You can use SNCF Connect (app or website) to check whether there are bike spaces available without actually booking. To book, you'll need to call SNCF.

For SNCF this is intentional I'm afraid. They removed the option to book pass reservations from their website and app about 3 years ago.