When you attach a gas can to a cooking stove top it takes two slots.

A frying pan or pot take up 4 slots, but can hold 4 slots inside. When you fill a frying pan or pot you can attach it to the stove combo (both the can and top) then it combines them and it only takes two slots in your pack. You now have:

Gas can - 1 or 2 slots
gas top - two slots
Frying pan or Pot - 4 slots
Contents of Pan/Pot - 4 slots

Total: 11/12 slots worth of items in only 2 slots.