The .ListIndex is like an array subscript into the ComboBox.
When .ListIndex is 0 - you're looking at the first value in the list
When .ListIndex is 1 - it's the second value in the list
When .ListIndex = 2 - the third value in the list, and so on.
So, in your situation, if ComboA has a value of 12.2, you need to know where in the list that 12.2 appears which would be
ComboA.ListIndex. Assuming that ComboA and ComboB are ordered the same, you want the next item in the list, whatever it's value happens to be.
ComboB.ListIndex = ComboA.ListIndex + 1
This increments to Index into the list, which I think is what you're after, it's not a increment of the Value, which is what you're doing.
Good Luck
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As a circle of light increases so does the circumference of darkness around it. - Albert Einstein