I don't see any problem with running duals on a 7720. We ran an 86' model 7720 for six problem free years with 20.8 38" duals. The combo of duals and RWA will greatly increase your 7720's opportunity for traction.
Will the duals fit behind a six row cornheadIJ Right now we have a 20 ft flex for beans but if we dont get freeze-up until late this year then we will be fighting the mud in corn too.