So you want to be able to SEE two (or more) images at the same time on your workspace. It is still possible. It is just that PSP2018 was set to the other way, by default. To change that (and I instruct everyone to do that as soon as they start using PSP2019) go to Window > Tabbed Documents and UNCHECK it. Then, you should be able to see as many images as you want on your workspace, and with duplicates like that, you can easily compare them.
Does that address the issue?