How do I plumb an ADU from a well?
No, you don't need a separate pressure tank. Assuming you are on level ground, getting to a 2nd floor ADU would be about a 10' (?) vertical rise. Water pressure caused by gravity is .43 PSI per foot, so if level land and 2nd floor, you'd loose 4.3 PSI in pressure. Which would  be roughly the same as between the first and second floor of a house.   Flow rate is determined by several things, most notable pipe size....and in your case, pump size and how much the well can produce.
If you are very concerned about this, bigger is better...for an ADU 3/4&quot; pipe would be fine, but 1&quot; would be better.
Also, most pressure switches that control pumps are adjustable meaning you could set it a bit higher to make up for the 4 PSI, but I doubt you'd need to.  Not knowing your specific setup, I can't be sure if it's adjustable or not, but most are.