Hmm, somehow I recall from similar Qs on tripadvisor that markSmith of seat61 (THE expert on train travels) advises to just book and then afterwards try to swap 1 seat, but thats more often for this EuroStar and Thalys (which in the end have very same owner=SNCF). The alternative would be to book 2 pass-seats and simply forget the seat for the ticket+booking-no idea how much extra this would cost-and decide yourself if thats worthwhile-for maybe 3-4 hrs.
Note that for using IR OVERtheborder it cost an awful lot more in surcharge and is also often much harder to get. IF you happen to be in DE now, DB should be able to arrange your supllmt booking-but only on direct trains to DE.
Let girl book really ASAP to check if there is still a reasonable price in high season.
Hint/tip: if girl is <28 also look at prices for 4-day pass=the cheapest-it may in the end not be that much more as a single advance-but again check the supplmts-plus that she then has 3 extra days (but only 1 IN DE) to make more travel-for nearly free.
Follow up-I am just sitting bored here in a tiny FR gare to wait a train.
Google like so often is best friend.
For some days in july DB still offers tix for 60-70€ all the way Avig-HH, with a very short transit in KA=you will miss this (in case you want to REServe this train nr 2). Thats indeed a lot less as 4-day IR.
And-again google helps: SNCF has FAQ-even in /en/ and indeed they advise to call (or use tr-agent, but an agent will never spend time for nothing to help you)- the nrs. It says No SURcharge=means normal rates and no hi-pay nr. Use /en/-this will come in the menu.
Is this for the direct train Avignon TGV - Karlsruhe? Just buy reservations at the DB ticket office or by phone (+49 30 2970). DB can sell tickets and reservations for it and should be able to book you next to each other.
Okay, we went to the DB "Reisezentrum" and booked her ticket and our reservations there. This was the easiest way for us. We got two seats next to each other in the tgv.
And we booked an ICE 2h later for Karlsruhe to Hamburg, so it won't be that stressful.
Thanks for your help guys! :)