I merged two segments, but then realized that they should have been left separate. I tried Ctrl Z several times, but it wouldn't undo the merge.In the end, I split the merged segments, but because the segments were cells in a table, the split segments ended up in the same cell, with a blank cell where the mistakenly merged segment had been.I'm using SDL Trados Studio 2019 SR2.
AFAIK you can't "unmerge" segments and the action is not undoable either. Splitting is possible, but I would avoid it (as I would avoid merging in general) as it can lead to corrupted SDLXLIFF files. It's worth getting the segmentation rules in the TM set up properly.
I wonder how Studio can manage merging two table cells - try generating the target file, it might throw an error. "Merge segments across paragraphs" is an option, but it is safer to leave it unticked. If I may say so, the logic of merging the content of two cells in translation is problematic to start with.