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.
Originally posted by Sundipped
^
This is the IB who scared Death and slugged it out with Big G.
Opposite or not, you still think Tiamut stomps this version?