You need to combine the mods together into one folder. So in your case, you'd want to copy everything out of the starbase mod's folder into HRGM's folder, then choose HRGM in the game. If the two mods don't use any of the same filenames this should go off without any further work, but if they do (say, both had a file named CustomPlanets.xml), you'd need to open both of the files with the same name, and manually copy the changes from one into the other (or just discard the changes to that file from one mod).