A session gives you access to both. GB1 (standalone) has a main mission of go gather ores/herbs.
HB has Gatherbuddy2 built into it. It does everything GB1 does, but even more, and better. Only problem is that it takes up more CPU/RAM space. If your PC can, then use HB->GB2.
So if you bought the Key that has 1 session, you could run one wow with either HB or GB.
If you bought the Key that has 3 sessions, you could run any combination of HB or GB (1HB 2GB, 2GB 1HB, 3HB, 3GB, etc) with a maximum of 3 windows open at once.
If you buy another key, you just add that # of sessions to your max allowed open windows of wow.