I think you can just install it all in one. The new Poser Pro installer that came in the Dev edition seems to have the game features as part of it, but they are hidden until you use your new Dev serial number and restart the program. It may be so future copies of Poser Pro will have the dev features built in, so they can be activated later without having to re-download all the installers again.

I uninstalled my previous Poser Pro 2014 before I installed the Dev edition. So it should work.

Daz Studio 4.8 and 4.9beta, Blender 2.78, Sketchup, Poser Pro 2014 Game Dev SR5 on Windows 8 Pro x64. Poser Display Units are inches