Degrees of comparison
FoundationThe superlative singles one out of a field. With only two in the field there is nothing to single out from — 'better' has already done all the work available.
Of the two proposals, the second is the better.
two things, so comparative
Of the two proposals, the second is the best.
superlative used for a pair
Where it bites
Whenever the sentence names a number. 'Of the two', 'between the two' and 'either of them' all force the comparative, and the paper puts the number early so it is forgotten by the blank.