The Hypro roller pump may be a bit small for your purpose. With most pump sales information...they are optimistic. You can usually get high volume or high pressure, but not both at once. I am not sure a roller pump is suited for wettable powders.
Remember that to use a second tank--you need a valve on the suction side--and another valve on the feed back line from the pressure regulator valve, (which is commonly also used for agitation). You don't want the two tanks to mix their ingredients. You definitely do not want any chance of applying a herbicide contaminated mix to trees...high risk.