One MBP (Call it 'A') connects to the internet by either ethernet or WiFi (there may other ways, but this is the most common way). Regardless, the Airport must be on.
In preferences, MBP A turns on Internet sharing.
MBP B then connects to MBP A by other protocol. So, if A is using ethernet, then B connects to A via Wifi, or vice versa.
MBP B looks for a network that is MBP A.
Little light on details, but I'm doing this from memory. Did around a little bit, it's all there in Preferences.
Good Luck