I was thinking on getting the Gallieo and use multiple transmitters to help monitor my girlfriends air. She is starting her open water certification in september and we are going diving in Bonaire in October. I figured it would be nice to have that option to help monitor a new diver.
You'be better off working on buddy procedures and communication between the two of you rather than relying on technology. Get into the habit of never being more than reaching distance away and asking for each others air on a regular basis. Also study gas management and use your SAC rates, Rock Bottom, and Thirds. Planning the dive properly based on your air consumption will reduce the need to monitor her air like that. And she should be checking her own air and telling you what it is without being asked.