Formula Help

Hello!
I'm struggling with writing a formula. Would love some assistance.

Here is the situation....
I have 150 product options for users to select from. 
They can select multiple products, but as we know, the multi-select text limits us to 100 options.
So I created two fields: Field A and Field B (both multi-select text fields)

I need a field that combines the two.
I'd like the formula to work like this:

If [Field A] and [Field B] are not blank, combine those two fields and separate the data with a semi-colon.
If [Field A] is blank and [Field B] is blank, return a blank field.
If [Field A] is blank and [Field B] is not blank, return the value in [Field B]
If [Field B] is blank and [Field A] is not blank, return the value in [Field A]

Can someone help me write that formula please? 

Thank you!!!