I don't know what others experience is, but we would agitate it really good in the holding tank and it stayed like that lots long enough to get it spread in the field.
Pig manure might start to separate sooner than cattle manure, but even then we never had any problems with it separating in the liquid spreader.
Now that you mention it, I think that there were two little paddle thingies on the drive shaft on ours. On our's, the shaft ran from the PTO, through the tank, to the spreader/impeller on the back end. We would take the impeller off to put the manifold and toolbar on. The newer tankers don't do that anymore. The bearings and seals didn't like the pig manure too well.
I sold mine years ago, so I don't have a very clear picture of what it looked like anymore. I just remember that the corn seemed to explode upwards when that manure hit the roots.