It would be awesome to be able to split the window vertically (side by side) and horizontally (stacked).
Right click on the tab at the top and click split vertically or split horizontally.
Same feature request in the Windows idea portal: https://support.royalapplications.com/support/discussions/topics/17000009157
Good news everyone! This is now on our roadmap for V4.
The first beta with support for splitting tabs is now available!
You can get the beta here: https://www.royalapplications.com/ts/mac/features-beta