Assign Airliner Seats
Python 2, 93 89 84 83 82 78 bytes



l=input()
for c in l[1]:l=[x.replace(c,c.lower(),c in l[0])for x in l]
print l


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Takes input as two strings. Prints two strings



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-5 bytes, thanks to Dead Possum
-4 bytes, thanks to Lynn