How can a private person put a SNCF train ticket on hold to combine it with its return journey later?

I usually travel to France for a weekend once or twice a year. Most of the time by train, the Netherlands to Paris by Thalys and Paris to wherever I go often by TGV and then the return journey a couple of days later.

For the Thalys it is important to buy the ticket the day, often the hour they become available, as the cheapest tickets sell out very fast on popular days.

I know people in the trade can put a hold on flight tickets and I have heard that some train booking sites also allow that, so you can buy both outgoing and return tickets at the same time for the price of buying at the time of the outgoing journey going on sale.

I mostly use one of the SNCF sites, but if it is only possible on the Dutch international or the Belgium rail site I can use those for at least the Thalys ticket.

Ideal would be to hold both Thalys and TGV outgoing and buy the four legs on the day the return journey comes available.

It seems this question is not clear enough. I am looking to select a ticket for the outgoing journey 3 months before the date, put it on hold for a few days and when available select the return journey and pay both tickets in one go.
Or, if that is the way it works, select the outgoing journey when they come on the market three months before travel and make a reservation for the return journey a few days later, (which will not be available yet) for the low 'early buyers' price.

I am often bound for the dates by events I travel to. But I know quite a few people who would rather travel an other date if they can not get both legs of the travel for a price they can afford. They rather not commit to the outward journey till they can be sure the return journey is also (relatively) low priced.

While I hope for a 'This is how you do it' I will also accept a sure 'This is not available for private people'.