It all depends on how many amps the total draw would be at the greatest load on the Power supply.
Before I say you'll be fine let me preface this by saying I'm not sure how many amps a single unit draws under a maximum load. So do so at your own risk. The Y adapter wiring has to be able to handle the load too. Depending on the gauge wire used it might not be able to handle the max load and could melt.
For instance, if all 8 pumps where to run at the same time the load on the power supply would be pretty high. But if you are confident that your load would never be more than just 3 pumps running full speed at the same time then I think you could get away with it. That's assuming a single powersupply is able to handle 4 pumps running at full speed and the units logic circuits at full load.
Keep in mind the KHD uses a minimal amount of amps to perform it's tasks so it represents a minimal load on the power supply. It's all logic curcuits and no real amp draw like a heater or a motor. This is the main reason you can get away with doubling up on one power supply when running the two together.
The pumps are the biggest draw on current and will use the power supply to it's fullest.
Lastly, read the labels. I don't remember if my Doser has a label on it saying how many Amps it's rated for and it's in a spot where I can't just lift it and look. But if it has a label, see if the Amps are listed. Then read the power supply and see what it says it's rated for. If the Doser is 1/3 of the max rating then you are good. Reason I say 1/3 is you should never push a power supply to it's maximum. So running it at 2/3 its rated ability would be safe and again assuming you are running all 8 pumps at max speed.