If they are both female and young enough, you might try just putting them together without keeping them apart, but make sure to supervise them at first.
Males are trickier, and the split cage method is probably your best and safest bet.
If you can put the gerbils in their own separate cages, try putting one in the other's cage and let them get to know each other gradually under your supervision. Put them together for just a couple minutes at first, and then increase their time together to ten minutes, then half an hour, then an hour, until they seem comfortable. When they start taking naps together, you can begin to house them together permanently. It largely depends on the individual personalities of the gerbils.