Is it possible to have more than one application running on the iphone accessing the GPS (simultaneously)?
Yes. For example, I use RunKeeper and the Maps app at the same time. Your app will need background privileges for keeping the GPS running. Here's the docs:
If your application needs to use the standard location service, you can declare your application as needing background location services.
An application should request background location services only if the absence of those services would impair its ability to operate. In addition, any application that requests background location services should use those services to provide a tangible benefit to the user. For example, a turn-by-turn navigation application would be a likely candidate for background location services because of its need to track the user’s position and report when it is time to make the next turn.