Hello Toaghoibou89
First of all: welcome to the SBB Community.
Not sure from where you have got the information that Geneva is the only Swiss city connected to Paris by a direct TGV service. Zurich and Basel do have them, too. And both of them can be reached by direct trains from Interlaken, not so Geneva, however. I would simply have travelled to Zurich all together.
Splitting the group is possible provided one day pass is issued on the name of one of the boys.
And, keep in mind the pass is only valid to the Swiss border, not beyond.
And, as a site note, carefully check that you will be staying in an AirBnB that is officially registered as such. There are many doubtful offers. If you are unsure, ask at the tourism office where they must be registered and pay the tourist tax.
Kind regards,
Roland