I've set up centrals on two trucks and I've learned more than I ever thought I wanted to know. For what seems a simple fluid power application there are a lot of little things that can bite.
You will need somewhere in the neighborhood of 15 GPM to run a spreader. I'm not sure what ratio a PTO runs at compared to crank RPM but you have to figure out what speed (RPM) you will be running at when spreading and work that into what GPM your pump will make at that speed.
You will also need a fairly big tank or a cooler to keep system temperature down. Even in cold weather my 20 gallon tank gets pretty warm on a long spreading run, I would consider that the minimum tankage.
You can tee both motor returns into one line at the rear. What I have seen done is to put one return hose into a tee with a male quick connect to hook to the truck and a female QC to plug the second hose into. I ran hard pipes and teed it to each side of the frame so I could keep my hoses as short as possible.
One drawback of running off a PTO pump is that you lose spreader function when you stop the vehicle. With my electrically clutched pump I can pull up to an intersection, or any spot that needs extra material, and just push the clutch and let the spreader lay down a heavy dose.
If that extra port you mention is a "power beyond" you will lose power to the spreader circuit when you raise the body or use the plow. I did that on my setup, rather than install a divider. A divider can be set to prioritize flow to the spreader and will keep spreader output constant through a range of pump input speeds. The drawback of low end dividers is that they make heat, which there is already too much of. I live with the variations in flow by trying to spread at a steady engine speed. It's kind of a pain losing the spreader when I tilt the body, but in most lots I can raise it enough for material to flow and leave it there for the entire lot.
Keep in mind that with any engine driven hydraulic system you need to calibrate the spreader at a cetain RPM and then hold that as close as possible when actually spreading. That is, of course, if you're trying to maintain a set application rate. That's more of a factor if you're doing long runs or streets.
I find that in my sites, which are relatively small, I spread at a given quadrant setting and usually run 2nd gear, high range at right around 2,000 RPM. On my truck that's #2 on the augers and 4 on the spinner side. If I have room enough to bring travel speed up I'll tweak the augers up to #3. If I want to lay a heavy band I'll drop spinner speed to #4. If I really want to cover area I can bring the revs up to about 3 grand and crank the spinner wide open and take the augers to 4 and cover 20' wide easily. I don't know exactly what my spread rate is but after several years I have a pretty good feel for what I need to apply and what works.
