Make sure the feature is enabled at the URL opera://settings/splitScreen first.
Then, ctrl + left-click the other tab you want in split screen besides the one that's currently focused. Both tabs being selected then might not be visually obvious though, but just go with it.
Then, right-click either of those tabs (the currently-focused one for example), scroll down and click on "Create split screen".
Or, instead of selecting 2 tabs, you can just grab the currently-selected tab down really far until you see the split screen UI and then drop that tab to have it split the screen with the tab next to it.
In the middle of split screen, you should see 3 vertical dots that you can click to exit split screen.
