Only restriction on AA's two-for-one certificate I used last year was dollar amount on first ticket; $329. Before it used to be $298 and it's now around $369. BTW the certificates were from signup bonus of AA/Citibank Visa/MC.
My brother used $99 companion ticket on CO and it didn't have any restriction as far as dollar amount was concern. I guess not all 2-for-1 deals are same...