If the attacker wasn't stupid, it would double it.

If he has an exploit for chrome, he is going to add the magic tag and let ChromeFrame render the code thus the browser is vulnerable.

If he has an exploit for IE, he isn't going to add the magic tag, IE's engine is going to handle the rendering and the browser is vulnerable.

The key is that the attacker can choose what browser the victim is using, thus he can target either browser.