RE: 7955 with a Y-Harness?
You just don't get it.
There is no "Y" harness that is rated at 6 amps, not if it has standard connectors on it. Even if you had no connectors and solder all the wires together, you still are feeding 2 servos through one standard connector coming out of one port on the rx. A standard connector is only rated at 3 amps.
The input of a "Y" harness is a standard connector that is rated at only 3 amps. You have just limited the current flow going through that one standard connector.
Now when the "Y" splits off to 2 separate servos, you have divided that current flow in half and you are down to 1.5 amps per servo. You have limited your current flow.
It is possible to push more current through a standard connector, but not without getting excessive voltage drops.
Limited current flow and voltage drops = diminished servo torque.
I don't know how to explain it any simpler than that.
Jim