I would be concerned that you are setting yourself up for a crash running an LPAR with a single disk, even if they are on separate controllers. You have no protection against disk failure in that set-up, so I would suggest that your server is not really capable of supporting more than a simple whole server.
Consider a disk failure, LPAR is down, end of. You are in a DR situation.
Is that acceptable? Protecting yourself involves spending money somewhere, I'm afraid. Do you have any external (SAN) storage available? Of course, you would need a method to connect to the SAN then.....
Other things to consider are ethernet cards. Do you have enough available for two LPARs? You would need at least two for each, one public, one management/data backup LAN etc. There is lots to consider before just launching yourself into running too much on a low-spec server.
I hope that this gives you something to think on. Feel free to come back with more questions though.
Robin
Liverpool/Blackburn
UK