Depends. Inbetweener is ALL over the place showings wise. In one scan he's ordering Death (a being more powerful than his masters) around like a dog. In another scene he's in a protracted H2H battle with Galactus with neither side winning. There's also a plethora of suspect showings (one time I believe he was owned by the Zodiac Key?).
If Tiamut has no opposite that can be invoked against him, he stomps Inbetweener into the ground. Even worse, if Tiamut knows how to split his dual selves asunder, it's lights out for the Inbetweener, opposite or no opposite.
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..even the outer hells are indifferent matters for they bow only to potent and archaic Nodens.
Depends, does Tiamut have an opposite IB can draw on to win? If not Tiamut stomps via physical beatdown (IB couldn't beat a just healed Galactus H2H). Or even more deadly (if he figures it out), he splits the IB into his component selves, ending him.
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..even the outer hells are indifferent matters for they bow only to potent and archaic Nodens.
Given Death's feats and going by IB's best then he wins. Going by averages then Tiamut wins. But the OP want's IB at his best only. So low showings don't count. So IB wins.