I have ML and SL installed as dual boot OSes and want to share a user's home directory (on an external drive). I wonder, however, if I rather should use different users for each OS as ML might write content to the user preferences which SL might confuse.
I strongly suspect that sharing a single home directory across different OS Xs will cause you problems. For example, after upgrading from Lion to Mountain Lion both Calendar.app and Mail.app had to upgrade my local data (i.e. calendars and mail boxes respectively). My hunch would be that the upgraded data would no longer be accessible from the older OSs.