Should "two" as a pronoun be spelled out in AMA style?

In AMA style, numerals are used for 1 through 9, but "one," when used as a pronoun, is spelled out. Easy enough. But now I'm looking at a sentence that says this:

Drug A enhances the performance of Drug B when the two are used in combination.

A colleague of mine changed "two" to "2," but I'm not sure that's right. "The two" seems to be synonymous with "(the) both," which is a pronoun.

Given all this, would "the two" be spelled out according to the same rule by which "one" is spelled out?

Answer

Your two should be spelled out. I cannot imagine any style editor anywhere demanding a numeral there. It's not a cardinal number in that context.

Use words to express numbers that occur at the beginning of a
sentence, title, subtitle, or heading; for common fractions; for
accepted usage and numbers used as pronouns;...

AMA Manual of Style: A Guide for Authors and Editors (10th edition)