Assuming you are actually keeping up with the support requests, and you're just wanting to avoid both people replying to the same email, any random division wil

Assuming you are actually keeping up with the support requests, and you're just wanting to avoid both people replying to the same email, any random division will do - for example, one answers emails that come in midnight to noon, and the other noon to midnight. Don't over-engineer this - since only one of you is technical, there's no chance both of you will be fixing the same problem. But definitely what chaosmachine said - you should be looking to set up a forum. If you are lucky enough to attract a "groupie" (you know, the super helpful enthusiastic user that is answering everybody's question), make sure you reward them (free invites, recognition, r0shirt, whatever makes sense).