When you want to add the same disk to a second VM you point it to the RDM that you've already created for the first VM. So going through the Add Hard Disk wizard on the 2nd VM select the option for "Use an existing virtual disk" browse to the Datastore where the RDM file is stored and use that. You can find the location of the RDM file by looking at the settings for the hard disk on the first VM. See attachment:
