Is there going to be a problem with the balls in the hitch binding up with all the tilt that 2 cylinders are going to give you? I am pretty sure that they would on my tractors. Something that is easy to check out before a lot of time and $$$ is spent.
Sorry Brian and Cyril for not responding to the earlier question.
In my case, I checked out the geometry of the 3pt (on my tractor) to make sure I could get all the tilt in either direction without snapping anything off. I used 8" stroke cylinders and can extend one cylinder fully while the other is fully retracted. You will need to do the same for your 3pt.
The concept of using a joystick to control the top and tilt is attractive, for sure!
I used independent controls for each cylinder because, I found, that sometimes you don't want the other side to move up or down when you only want to move one side. For example, if you are cutting a deep gutter with one side down and the other up, you have your 3pt position in a certain position ... probably up a little. As you start to level the blade to feather out the gutter cut, you will find the blade coming out of the gutter cut, above the ground. You would need to use the 3pt position control to place the blade at the desired level, back on the ground again. Not to say this is a bad thing, because I have to do something similar when doing a severe tilt.
But I like the ability to move only one side when I want to.
In other words, say I'm dragging with the blade level, and I have set the position control for this cut. Then want to cut deeper on one side only. I lower that side link and that side cuts deeper. In that case, I don't have to adjust the position control again. The other side does not come up ... it stays where it was. With the single control, one side always comes up when the other is moving down. So you would need to lower the position control at the same time as you start to tilt the blade. And being that our tractors usually allow only one valve to work at a time, because the flow so small compared to commercial vehicles, trying to work the two valves at the same time might become difficult. Anyway, since I don't have my two side links plumbed like that, I really don't know. What I do know, however, is I can move my side links separately or simultaneously by feathering since they are controlled by the same valve.
One thing is certain though, regardless of which way it's plumbed, having two cylinders ... one retracted fully and one extended fully, you get a heck of a lot more tilt. And it is done in an instant instead of having to get off the tractor to manually screw up or down the adjustable mechanical side link. It would be great if jenkinsph would make this mod to his tractor. Then he could tell us how it really works out in real world use and how he likes it. Like I said, it sounds like a very attractive idea.
. You said that the cylinders were "with check valves" - not sure I understand this either. 
