Gary,
The two grapples are operated off of one hydraulic supply line, and one return line. It operates the exact same as a single grapple, only difference is that there is a Tee section put in the main supply/return so it branches out into two different hydraulic rams.
As an example, if I operate my 3rd function valve, and say I want to close the grapple half way (from fully open), I press the magic button to close the grapple half way, then let up. The two grapples will close the same amount because the exact same volume and pressure of hydraulic fluid are going to each grapple arm. If I got off the tractor, I could grab say, the right grapple arm, and physically push it down, to completely close that grapple arm, however, the left arm will now open fully, since they are Tee'd together, the fluid has to go somewhere.
The same thing happens when you pick up one single large boulder or something, lets' say about 2' in diameter for the heck of it. If you pick it up in the middle, you would be grabbing it with only the insides of each grapple (this is where a single grapple arm would completely grab the item, and hold it more securly than two arms) on the two grapple setup, both arms would close the same amount and the item would be centered between them. However, if you now drop the boulder or whatever you have, and pick it up with one grapple arm, it will work like this. With both arms fully open, you manuever the tractor to pick up the boulder with the right grapple arm, as you start to close the grapple, both arms will close together, then once the right arm starts to close on the boulder, it will stop, and the left one will continue to close fully, since there is nothing there to stop it, then once it is fully closed, the right arm will then close tightly on the boulder, and you can pick it up.
Hope that makes sense.
