Where more than one rope must be made up on a shoreside cleat or bollard, always pass the loop of the second and subsequent lines through the loop that was there first. The geometry of this is hard to explain, but the result is that no matter what loads are on the ropes, when you slack one away you will always be able to lift it clear.
This seamanlike solution is a far happier answer than wrestling with the made-up bight of a single line doing duty as a pair of springs.