Sounds like you need a longer chain. Be careful when in the big/big combo (not recommended anyways because you're cross-chaining, but it happens, right?). Your rear derailleur could get sucked up into your cassette.

Not that I know from personal experience or anything.....



On my tour last summer, I had this happen. Needed one more link in my chain. Since I couldn't do that, we had to lock out the derailleur so I couldn't use the largest cog on the cassette.