Are you using the ‘FREE PRODUCTS’ option or the ‘BOGO’ option? With the ‘FREE PRODUCTS’-option it will work if you set the MIN QUANTITY OF MATCHING PRODUCTS to 2, and enter the same PRODUCT on the usage restrictions tab as in the ‘FREE PRODUCT’-field on the free products-tab. This only works for 1 kind of product per coupon.

With BOGO you would need to use a snippet in PHP which I can provide.