How can I select two files, one in each panel; to compare them with third party software?

I try to compare two files, selecting one in each panel; but Opus selects only one of the two.

Directory Opus: 13.1.1 Build 8797 x64
OS: Windows 10.0 Pro Build 19045
Hardware: Intel i7-1065G7 • 16GB RAM

Works here:

If the two files to be compared are in the same pane, there is no problem selecting them with Ctrl-Left Button; but if each is in a different pane, it does not select both.

Do you mean it literally deselects the file on one side as soon as you click on the other, or that the compare operation is only using one file?

What are you using to compare the files? There are some buttons on the forum but nothing built-in to do that.

That's right, it literally deselects the file on one side as soon as I click the other side.
I use Ultracompare

I like the theme you use. Is it available for download?

Thanks! Not currently. Will have to narrow down some extensive customization first.

Is it still deselected if you click the status bar or folder tab to make that side active again?

(The color blending settings in Preferences may be set to make the selection color for inactive file displays so faded you can’t see it.)