Doesn't the flow on an open center system go through all of the SCV in series?   If they are in parallel I would think any time you tried to do two things at once it would not work. 

It seems a little odd that DTAC would say that running a hydraulic motor on a open center system will ruin the pump, and then they go on to recommend running it on a PTO driven system instead.....such system almost certainly being an open center system as well.....

I sure hope we can run a vacuum planter with an open center system because we just spent two days plus putting one on our 35 to run our 1200 CIH planter fans with.