Perform a small search with a word that you know is in both the series names.
Hold the CTRL button and select both series.
Right click and use “Merge to:” and select one of the series.
If it doesn’t work what you could try is (as a test) edit them (both of them) and give them unique names… then merge them. Then rename the remaining item to what you wish it to be named.
If it still doesn’t work.. can you show me in a video? You can record your screen via this website: