Fairly sure it is 10/20 per train

It is per train-and it can thus also be 10+20.

And-search! there are some long-dist trains that run around Paris and may offer you a direct trip-at other times. Or f.e. take Lille-Bordeaux, then free TER onward.
However, when last yr/end of summer I was in this area and tried some comboś out-I noted that some of these long-TGV even charged 30 extra. No idea if this was perhaps due to very busy times and short notice then.

I have also had an odd connection of 2 TGV show as €10, not sure why or if the booking would have completed as I was just searching.

I have also once seen that and I've read another report from someone else. Not sure what the logic is behind that, i.e. when SNCF only charge one reservation fee.

Yeah, not sure either. I can’t recall which route it was, perhaps there are some instances where fixed connections are counted as a single journey or something.

I guess the only take away is that if you find one, take advantage of the cheaper reservation.

Hi! There are several results for this route (Via Massy). The TGVs in every case in this route cost around 22 euros, per train.

If you make the reservation elsewhere (SNCF, b-europe or other) than the Interrail/Eurail website, the cost is 10 EUR as long as there are pass holder seats for 10 EUR avaliable. Then the cost is 20 EUR. 

I think that it is bad that Interrail/Eurail charges 20 EUR (+ 2 EUR booking fee) for seats that should cost 10 EUR + booking fee. 

At SNCF Ticket-Machines, the prices for Reservations cost less some time, and also there is a bug and it shows 10€ for two trains. 

I'm not sure this is even compatible with general consumer law.