We have 3 CAS+HUB and 4 MBX Exch2010 servers. All are on SP1, RU5. We are scheduling to install SP2 on all these servers.
Do we need to install SP2 on all servers in the same day or can we install first on CAS+HUB servers and let it be there for about a week and then install it on MBX servers?
Yes you are able to schedule SP2 install as above...Personally i would do one box at a time and see what errors you experience before carrying on with the next one.
On a Client Access Server (CAS) there is a requirement to have IIS6 WMI compatibility installed.
Here is a nice article regarding the SP2 install: