Socialization is no guarantee that they will accept each other. I've had pairs that stayed separate their entire lives even after weeks of split cages. After some point, you just have to accept that some won't bond because the minimalism of the split cage method is just too hard on them. Having too many things in the tank becomes a source of contention so it's really important to have just the bare minimum in split cages. You can generally tell if gerbils will accept each other if when you switch their sides they don't react to the strange smell. The other indicator is dominance which you can tell by excessive scent marking, which the submissive ones do less of. A gerbil that stretches his/her neck out flat to the ground when being greeted is the submissive one. The one grooming after this meeting in this scenario is dominant.