What are the models and manufacturers of the mice?

The reason that they're interfering with each other is that both are operating on the same wireless channel. It basically means that the base stations have no way to tell which mouse the wireless signals are coming from, so it tries to process both. There is probably a way to change the channel or frequency on which one of the mice operates - you might be able to find something in Start > Control Panel > Mouse.

It'll be easier to help you out once we know the models of the mice though.