The "two-six" heave comes from the order for a guns crew to "Heave" on the tackle required to run the gun out prior to it being fired. As previously stated the RN relied on numbers and calls on a bosun's pipe.

"When the men were in position the rating in charge would give a command to ready them,followed by two,six heave,this was repeated as necessary to complete the task.
The effort was co-ordinated without the need for a song."

The command to ready them was the "Two-Six" given by whoever was in charge of the evolution the "Heave" was the response given by those on the rope, or whatever was being pulled.