I wouldn't bother, I've never seen a report about C/Ku combined LNBs that doesn't say the Ku part isn't crap. C Band works OK on them but Ku is rubbish. If you had a larger dish I'd say bolt two LNBs next to each other with Ku cented on the focal point but with such a small dish you really need as much performance as you can manage. I'd fabricate new support arms and give each LNB it's own mount and feed to make changeover as quick as possible.