You don't have to use them at the same time. You can save a scene in Poser as a *.pz3 file, however if you import that into Carrara it needs much time and some poses are not looking the way as they do in Poser.

Much easier is it to use just your Poser runtime in Carrara where you will find it under 'Content' if you add the runtime in the data menü. This way you can set up your Carrara scene with you Poser stuff, except the lights and cameras!

If you want to export Carrara stuff into Poser you have to export them as *.obj (works good) or *.3ds (works bad), you cannot export them as a scene like you can import Poser scenes into Carrara.

This way it not only works for Carrara 7 and 8 plus Poser Pro 2010 but also for prior versions of Poser.