Yes, this can be done, but only with Mac OS X Server.
This is virtually impossible to do without either Mac OS X Server or "syncing", which you've specified you don't want to do.
With Leopard, it's relatively simple:
http://nunojob.wordpress.com/2007/12...ent-partition/
With Tiger, it's a little different:
http://maczealots.com/articles/home/
Be aware that "sharing" a home directory between different versions of Mac OS X can have unexpected results. Some applications may not work, and it may even render your user account unusable on either system, since there may be slight differences in the way Panther and Tiger handle your home directory. I would do a full backup before attempting any of this, and, if it doesn't work, I'd be prepared to need to reinstall the operating system on one or both of the computers.