Why not just use the drag and drop? Create a new folder “MAIN” and Drag SUB1 to MAIN then drag SUB2 to MAIN. This will move all the sub folder in the SUB1 and SUB2 as well.
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mvoe /-y <source folder> <newly created folder>
now save the file as .bat extension with a suitable name. The /-y helps to prompt, if there are duplicates. Keep in mind, move command is using for delete the original and create new ones.