I see two meanings here, a higher brow and a lower brow variant on the joke. The first is the sexual connotations discussed in the comments to your post: of two extra cheeks and showing her self off. At first I thought this was the only meaning, until I though of the phrase "two faced"1.
Twice as many faces, twice as many cheeks (you do the math!) I feel as though the higher brow one like this is much more likely to be the true meaning, judging from the (apparently) sophisticated context. I did google this to find some kind of evidence to back up my thoughts, but only came across a TV program review and this question again.
To surmise, I think that the joke means that she's two faced, but it's ambiguous.
1Two faced is defined as a person being deceptive or sly, quite possibly ready to betray people. From Google:
"insincere and deceitful."
Some synonyms, too:
deceitful, insincere, double-dealing, hypocritical, back-stabbing, false, untrustworthy, duplicitous, Janus-faced, deceiving, dissembling, dishonest; More