Oh dear, this is disappointing. Actually, somehow, I managed yesterday to get two documents WITHIN the TextMaker viewing screen, which was a sort of "next best" option. Unfortunately, today, I can't get it back in the same way. In fact, I wasn't even quite sure at the time how two (and even more!) documents were suddenly opening up into the same Textmaker screen. (I have a wide screen that can take two documents side by side.)
Lack of this feature will stop my husband from buying the software. I think he will now carry on with Microsoft. It does make serious editing work much easier.
I also lost a lot of work yesterday in Textmaker

and am not quite sure how, or if it had anything to do with having two documents within the same Texmaker viewing screen.