which way is seat facing on TGV duplex (Paris-Barcelona)

You do not know. Car direction may change even en route. If you are booking two seats, consider choosing seats opposite one another.

Two basic designs of stations some are straight through, others the train pulls into a dead end, then reverses direction to get out. Even stations that appear to be straight through, the train may switch tracks by reversing directions.

So as the previous answer indicated, you just can't know (except possibly initially if you can find the pertinent train makeup chart), but direction can change during the trip, sometimes more than once.

Travel direction does not change from Paris to Barcelona, and indeed there is no way to know the direction in advance. The safest bet is probably to request seats around a table. In that case, 1st class is best, because you have tables where 2 people sit face-to-face. 2nd class only have 4-person tables which are frankly a bit cramped for a 6-hour journey!
Also, if you suffer from motion sickness, upper level sways more than lower level.

someone said couch 11 is towards Paris side, with the train coaches 1-10 splitting off in Nimes and 11-20 going to Barcelona. So, if there's a Paris said, is it likely that it's a "pull out" station (Gare de Lyon) and the top of the seating chart diagram would be the left side (headed South)?
And, yes, I'm going to try to get the 1-1 front/back facing seats--but you're not allowed to choose. You have to put into shopping cart and then book more seats, until you get what you want (tip on seat61).

Yes, Gare de Lyon is a pull-out station.
Try first class; you can choose your seat in SNCF first class TGV. Is this a French train or a Spanish TGV?

I'm curious to know why you can't ride backwards because you are the second person to say that on this Forum in a week.

for this train, not allowed to choose seat in 1st class either. The backward motion is not good-it must disorient inner ear? who knows?No issues with swaying, bumping, etc. if front-facing. Luckily, doesn't matter to spouse.

Where it says preference, choose face-à-face. In 2nd, there are only four-seat face to face, which you'll have to share and preference goes to families. In 1st, there are 2-seats, duo face-à-face where you and your husband have the two seats to yourself.
To increase your chance of getting a duo, accept either upper or lower seating. We always choose duo face à face when available.

If there is a mirror or window reflection, as I had in the tram yesterday, it can be confusing. I looked out a different window.

Perhaps because so many people in Europe ride public transportation from the time they are infants, they are ok in any direction.

in 1st class on SNCF trains within France you can pick your seat, but when I bought Lyon-Barcelona tickets 1st class recently, there was no option to do so. It's a joint service with RENFE so this may just be a difference between the services.

Hi @elaine - did you ever figure this out!? I am in the process of trying to!

so, in rethinking this, if coach 11 (1st class) is at the "Paris" side, and it's a pull-out station--doesn't that mean that if 11 is closest to Paris, then 12, 13, 14, etc. are "closer" to Barcelona, so the train runs in reverse order, with coach 20 being at the front of the train headed to Barcelona?

The best thing to do is see where your seats are on board. If the train is not full, the car attendant can check seating on the spot and help you select other seats. That is what we did on one of our trips. We were at a four-seat table top which we did not want, and we asked for duo seats facing the direction of travel. He accommodated very quickly. This was in first class though.

On our return train from Strasbourg to Paris in December 2021 we were suppose to be facing the direction we were traveling. When we got onto the carriage, our seats were facing the opposite direction. A few other passengers noticed this and were not too happy about it. Nothing no one could do about it.

Enjoy your trip.