Was one out of the cage for a while and just reintroduced? Its almost impossible to reintroduce gerbils once they have been apart for more than a day. I've had tons of gerbils in my lifetime. Some did much better living on their own. Your solitary gerbil did not die of lonliness. I had two that were born together and never seperated but started fighting suddenly and tore each other up bad, I had to keep them seperated for the rest of their time. This might be your only option as you certainly can't let them alone with each other.