Hey there!
As an active breeder, you can remove a gerbil from its cage-mate, breed them, let them have a litter or two, and then once the last of the pups from the last litter are weaned, return that gerbil to its cage-mate from before. But, as stated, it requires the Split-tank method of introduction.
This can take anywhere from 3 to 6 weeks, give or take the personality of the gerbils involved and it is harder with adult female to adult female than it is with adult male to adult male or adult female or male to young pup.
You cannot, however, in the case of females - remove a gerbil, breed them and then return that gerbil pregnant to their previous cage-mate.
Where its been discovered that a gerbil's memory is much longer than we give them credit for -- as I actually did an experiment, with males mind you, where I took two males who have been cage mates for least a year. I removed one and kept him away for 1 week, and then returned him back to his cage-mate without split tanking. His cage-mate seemed to remember him and after a good 20 minutes of scrutiny and sniffing, everything was fine.
Now.. I did this only once AND I didn't subject the removed male to ANY other scents.
I didn't place him with another gerbil, pup or adult, nor did I place him with a female.
Where this reintroduction was a success, I wouldn't recommend doing this without being an experienced handler who's had a long history with caring for gerbils.
I certainly wouldn't say either that this would work for ALL gerbils. Definitely NOT, without more careful and cautious experiments.
So it really isn't a case of if the cage-mate you removed them from remembers them in order to return them, even if that female mated and was returned to her cage-mate the very SAME DAY that you removed her to breed. It's mostly because of the kinds of problems this can cause::
1.) They will have the scent of a strange gerbil on them - whether male or female - and this can cause an issue which would lead to split taking.
2.) In the case of females, unless you remove the dominate gerbil for breeding, returning a pregnant submissive gerbil back to her cage-mate can cause the dominate to either punish the submissive female or, worst case, cause the dominate female to harm the litter of pups when they are born.
3.) Even if it IS the dominate female who you breed, returning her to her previous cage-mate can still risk harm to the litter of pups from the other gerbil.
It is possible that the other female gerbil would help with the litter because gerbils are a matriarchal society? Its very possible. But I haven't experience with this and perhaps someone else who does can give some insight. To me, it isn't worth the risk to the litter.
I have had numerous successes with taking males from each other, breeding the males, and then successfully split caging them back to their original tank mates.
But I always split tanked them and never just "put them back with their buddy". Again, the males that mated would have a strange female's scent on them.
I hope this has been helpful!