I was on dual rom (Sammy and Miui) previously and i have tonnes of problems. I have some immediate freezes and also the so called slow bug, where the phone goes into stand still. Whatever extweaks settings i have done does not help.
I eventually took the plunge and do a clean re-install of my phone (both Sammy and Miui). I also re-install all my programs instead of using backups from TB or from Miui.
Right now, I do not have freezes or slow down anymore, and the phone is working mighty fine.
Galaxy S II
Primary Rom - DianXin OS
Secondary Rom - MiuiAndroid
Kernel - Dorimax 4.0b