How did this fail, what sort of error messages etc?
I haven't tried this myself firstly. However, I can't imagine this working, most operating systems will write various state data and lock various files when running, this could be quite dangerous to run two virtual machines from the same disk (image file). The virtual machines could want to write to the same file at the same time and this would cause issues. Already Xen prevents you from running virtual machines where the image is mounted in other domains or at least dom0.
If you have some sort of read-only image file such as a CD-ROM or linux distro which performs writes directly to RAM instead of the disk then this may be workable.
Best thing to do if you want to run multiple instances of the same VM is to just clone the image file using dd.