Richard:

Can you find the point where the 1/2 inch water supply line under the house separates into two 1/2 inch lines; one supplying cold water to the house and the other supplying water to the water heater?

If so, then I'd replace the 1/2 inch copper pipe up to that branch with 3/4 inch copper pipe. Then, at least, you have full hot water pressure to one fixture or appliance, and full cold water pressure to another fixture or appliance at the same time.

However, as it stands now, that 1/2 inch pipe under your house now is a bottle neck, and it's most of the reason for the lower water pressure whenever any two faucets are opened.

Obviously, a better situation would be to have 3/4 inch copper piping throughout your house, and tap off each supply line with a 3/4 X 3/4 X 1/2 tee at each appliance or plumbing fixture so that any two appliances or plumbing fixtures can have full flow to them at the same time. That way, one person can have a shower without fear of being scalded if someone else flushes a toilet somewhere else in the house.

Redoing the water supply piping with Pex is something I've never done, but I've been told that it's much easier than using rigid copper tubing because the Pex tubing is flexible, so that you can run it around corners instead of having to cut the tubing and use an elbow.