Does Ultra Instinct have a larger multiplier than Potara fusion? So in Dragon Ball Heroes is pretty clear, Goku Ultra Instinct is able to overpower Cumber, something Super Saiyan Blue Vegito Kaioken times 50 wasnt able to do. So in Dragon Ball Heroes Ultra Instinct has over 50 times a larger multiplier than Potara Fusion. 
But how about the canon universe? Are there hints or statements which shows which would have a larger multiplier, Ultra Instinct or Potara fusion?
The Multiplier for Ultra Instinct is definitely greater than the Potara fusion Multiplier. We know this as the SSJ Vegito for the Buu Saga, is nothing in comparison to MUI Goku. There are several other examples indicating the same:SSJG Goku being stronger than Vegito in the Buu Saga(MUI is several times stronger).UI Omen completely overwhelming SSJ2 Kefla and MUI is even more stronger.The Potara fusion is also dependent on the characters who fuse and I think it is safe to assume the multiplier always stays constant. Disregarding the power of the fused characters, the potara fusion multiplier will never be > than the Ultra Instinct Multiplier. It wouldn't make logical sense for a potara fusion between Hercule and say, one of his students, be able to go toe to toe with a god of destruction.