Replacing the DP switch with a 2 gang 2 way switch is likely to be OK, but from afar we cannot tell if there may be other problems this may cause in the wiring. It does seem as if the live supply for the fan is not fed from either of the switches, but possibly from the fan isolator.
However to be able to categorically state it would be OK would not be possible at forum level.
Fair enough. Will try it then