LoNet GSM/GPS with two host devices simultaneously?

I’m sorry for the subject, I guess i’ts very unspecific but I didn’t know how to put it better.

I have a project in mind - a computer for my car with lots of functions, powered by a Raspberry Pi. For using GPS and Internet, I find that the LoNet GSM/GPS breakout board (http://www.seeedstudio.com/depot/LoNet-Mini-GSMGPRSGPS-Breakout-p-2183.html) is a very interesting candidate. I think I will integrate it into my project.

I also want to integrate some kind of anti-theft tracking system, in other words, a small microcontroller like an AtTiny, connected to the LoNet breakout board which will send the current time and coordinates of the car to a server in my home network every five minutes or so. Of course, this microcontroller should be always on and always transmit, regardless of the car computer’s power state.

My question is: as the LoNet breakout board seems to use RS232 communication, I guess it’s usage is exclusive - either the Pi can access it or the µC. Is this true or can I link all devices on the same Rx-/Tx-lines? And even If I could do it physically - can the LoFi be really used by two devices at the same time?

What would be an elegant solution instead? I have some ideas, but I’m not quite satisfied with them and I would like to hear your opinions and expertise.