Open multiple clients for tax preparation or review.

You are making sense, but it sounds like a recipe for disaster for some folks.  If they do that, another person is going to come along complaining that they had multiple client files open and then they accidentally started entering data in the wrong client's return.  Some things are better left as is.

While working on a client's return, another client calls regarding return under preparation.

Tell every client they deserve and receive your undivided attention, even if not physically present when you are working on their returns, and tell the other client you'll call back later.   

So many times Ive entered things in the wrong year, for the same client, when having 2 years open at once.   I dont want to have 2 different clients open on my screen at one time!

Pro Series allows you to change the PRIOR year's program to have a different color background to having two years open are less worrisome.

It takes a little practice to realize it.

The Options Tools option to change the background color does not appear until NEXT year. So you cannot change the 2024 background until you have installed 2025 software.

Yea, but if you've got 2 different prior years open, they're the same.