Confirming Xermo's post. If you didn't buy the double-packs previously, then you're stuck buying at the single-console rate.
That said, as a potential alternative, if you have a friend, or friend of a friend, who has recently acquired the 3DS version (or doesn't have the DLC yet), offer to split the cost of the dual-version. Saves you a few dollars.
Examples:
/
/
(DLC Veterans)
Buying alone is $4
Buying a double pack is $5
Splitting saves $1.50, final cost $2.50
(DLC Newcomer)
Buying alone is $5
Buying a double pack is $6
Splitting saves $2, final cost $3
/
/
(DLC Third Party)
Buying alone is $6
Buying a double pack is $7
Splitting saves $2.50. final cost $3.50
Still a bit higher than just getting the double pack yourself, but better than nothing at this point.