What OS you run isn't going to matter when it comes to your carrier's ability to track your location -- they can do that on completely dumb phones as it is a function of radio triangulation. The point of Graphene is to prevent you from leaking data to Google, and to prevent apps from leaking data from one app to another, more than anything.

As for multiple SIM's, someone else will speak better to that than I can. Presumably you can choose which you're using when making calls (and using data) but I don't have direct experience there. The SIM is basically your credentials to access each network, so managing them is going to be how you manage what carriers get.