In Peking Express, another similar reality tv show, they do that for one leg.
Production says the couples, and it's just for one leg. The new couples start the leg with their new partners, they do the whole leg and then they must return with their partners a few km before they step to the Pit Stop.
They normal swap the people to force some love affairs, or some rivalries between the couples. For example, if Cody couldn't stand Connor, they would put them together. lol just an example.
I don't know how they are going to do that in TAR, maybe they put papers and they must pick one based on luck, the couples choose the swaps, or production choses.
I don't think it's a bad idea, it's always a fun leg to see and there's always drama and funny moments. And they also can rest from their respective partners. xD
We'll see.