No pressure to upstairs bath

Just recently I acquired a home built in the sixties. When they built the home, the contractor put in 1/2 copper pipe everywhere (No 3/4 copper). There seems to be very low water pressure upstairs. The previous homeowner had two hot water tanks installed within the past 10 years. Basically one was for the upstairs bath and laundry, and the other was for the main floor baths and kitchen. However both of these units are linked together. Any ideas as to why there is very low water pressure? Cold is okay, although I would like more, and hot is just terrible. I was thinking that either the hot water tanks are too small and not powerful enough to send the water up to the second floor or instaling 3/4 copper would help. The main line is connected to a city water system and lake, although I do live in a small town. Any suggestions as to where to start to solve this headache? Thanks in advance.

If your pressure is okay on the bottom floor, you may have a restriction somewhere, eg., the hw tank that feeds the upstairs bath and laundry may have a restriction in either the incoming or outgoing line. Other possibilities: Your faucet aerator may be plugged, your shower head may be plugged. If you have old, galvanized pipes, they may have deteriorated and partially plugged.

One thing is for sure: the size of your HW tank(s) have nothing to do with the lack of pressure. A HW tank is not a pump. The cold water entering the tank pushes out the hot water at whatever the house pressure is.

Have you checked the shut off valve(s) at the water heaters? A partly closed valve would cause this problem.