Janet is the......of the two girls.
a. taller b. tallest
I think both 'a' & 'b' are OK. Am I right?
In 'Practical English Usage', M. Swan says "
groups with two members:
When a group only has two members, we sometimes use a comparative instead of a superlative.
I like Lily and Mia, but I think Mia’s the nicer/nicest of the two.
I'll give you the bigger/biggest steak: I'm not very hungry.
Some people feel that a superlative is incorrect in this case.
Also, in 'A Comprehensive Grammar of the English Language',
'The superlative is often used for a comparison between two persons, items, etc , though this is avoided in careful usage where the comparative [3a] is preferred :
He is the youngest (of the two brothers). 
He is the younger (of the two brothers). [3a]