If they share a common ground plane you are in for some difficulty in trying to prevent the second part from heating too much.
What happens to the underfill material when it is softened by being heated beyond the 160C? Does it harden up again, or become useless?
You could remove both parts, and help assure that you are not causing any compromise in the second component by replacing the first one.
You could "nuke" the one you are removing using a heat gun with a focused tip, and keep ahead of the heat spreading, but that provides you with no means to resolder the new part down without facing the same issues.
Tough spot. If cost were not an issue I would replace both parts. If cost is the barrier, well, you're up against it.
Most important though, is the reheating of the underfill product a real problem? Maybe you should go with it, and flux both parts during the process of removal, and the second part during the replcement of the first part.
Good Luck and Happy Holidays
`hege
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