You could hold with both hands sure, as long as you accept that you have to take either a blend of the offset between the two hands or largest / smallest hand motion to run the climbing off of.
Climbing movement mode is just a direct input movement mode, you tell it where to go and how much. All of the movement logic for it in the example is in BP and is in CheckAndHandleClimbingMovement function in the character.
I don’t think you will actually like the end result of what you are attempting, it will feel really weird to the end user. However its perfectly possible to enforce the original grip X/Y and only increment the Z.