The "distance" in between the hosts should be maximized, 
being a seperate
routed networks, seperate storage etc to have the risks 
minimized.
Personally, I would not mix the two, unless having spent a LOT 
of time in
isolating things, just as you would do with two physical 
hosts.
--------------------------------------------------------------------------------------------------------------
 
Well even though they are on the same machine, they are indeed on 
seperate routed networks/bridges (Public ones have public IPS, internal ones 
have private IPs). The VMs will share storage though, however I'll remember to 
zero the LVs before giving them to customers.