I have a pretty similar setup one vCenter for Servers and vCenter for View, they also both point to the same DB server but I didn't have any problem upgrading them from 5.0 to 5.5 and just installing SSO alongside the vCenter upgrades (still have the server as opposed to switching to the appliance) and they both point back to the same DB as before.
I did decide not to link them together in SSO though for security reasons and organization of our people (the View group is different from the server group) so it just kept things clean for both groups to see what they were used to seeing.