I'm happy to run tubes with many patches, but to date have drawn the line at patching a patch. I guess if the composition of the top surface of the patch is similar enough to the tube's surface so that the next patch adheres properly, then there's no reason it wouldn't work. If the puncture is near the edge of the first patch, there might be some concern about sealing and leaking along the edge of the first patch where it meets the tube.