Lorerim - Dual Power Attack/Horned Lord

Trying out the Force of Nature transformation magic in Alteration. The promotion when you transformation states a dual power attack will cast Mark of Hircine. However, I can't seem to perform a dual power attack. Right Power Attavk and Legt Power Attack work fine, but nothing when I attempt a dual.

The spell description also states that the form can cast the spell in my other hand. However, I can't seem to perform that either.

Any help would be appreciated. Thanks in advance.