Well, you place them against eachother with the screwholes alligned properly and then you screw in screws that are long enough? And if you want to make two cables into one you do as always:
Strip them and reconnect them correctly to eachother and then put a new plug on the now newly made end.
(wow, you gave me a very good idea. Not the putting the two fans on eachother but the stripping of the cables and then connecting them so they only take one wire/plug/connection)