Hello. I volunteer in animal rescue (fosterer until we can find animals forever homes) and I specialise in Gerbils 
We bond all the time, and we never let Gerbils be rehomed as singles, or owners come forward with their own singles for bonding. As ever, I'm highly dissapointed that the dreadful Pets at Home continue to split bonded Gerbil families and leave singles, thank you for trying to save them.
The success rate is about 70%, and its really important to follow the split cage method. It sounds like you have plenty of room in your current tank. Some people will ignore split caging, and claim success by just putting them together, and while it "might" work in the short term, chances are it won't and result in a nasty fight.
SPLIT CAGING
Separate the tank with a wooden frame and mousewire.
Setup about an inch of substrate (finacard etc) so they can't burrow. They must focus on each other.
No toys, nothing they can be territorial over.
Likewise, no food bowl, scatter feed (which is the best way to feed them anyway as it encourages natural foraging).
Fix water bottles to the glass with industrial strength velcro.
Swop them over to a different side 3 times a day ,(always an odd number so they sleep on a different side each night).
No interacting as they are very scent based, and leave for about 2 weeks before raising the barrier.
At which point, hopefully you've made two lonely gerbils very happy.