Just thought of a possible application of the double-Tin combo: biofeedback or, to quote TVTropes, Master Of Your Domain. Such a double Tineye could spend ages burning tin Allomantically and storing it Feruchemically. When she splurges her touch-and-hearing-Tinminds, she'd presumably have inhumanly enhanced awareness of bodily processes; perhaps hearing and feeling every artery, every vein, every muscle and nerve in her body at the same time. What you can detect, perhaps you can control. If this is the case, the double-tin Twinborn could learn to slow (or speed - or stop) her own heart at will, suppress wound-bleeding via vasoconstriction, warm herself in cold temperatures via vasodilation, generate surges of adrenal fight-or-flight "hysterical strength" at whim and much much more. Perhaps they'd even be able to heal faster (not like a Bloodmaker or even a Thug but a bit faster than normal nonetheless.)
tl;dr: A double Tin Twinborn could be like the Bene Gesserit from Dune or the Dunyain from Prince of Nothing.