In this paper, we study the duality theory of the multi-parameter Triebel-Lizorkin spaces Ḟ α,q p (R m ) associated with the composition of two singular integral operators on R m of different homogeneities. Such composition of two singular operators was considered by Phong and Stein in 1982. For 1 < p < ∞, we establish the dual spaces of such spaces asWe then prove the boundedness of the composition of two Calderón-Zygmund singular integral operators with different homogeneities on the spaces CM O −α,q p . Surprisingly, such dual spaces are substantially different from those for the classical one-parameter Triebel-Lizorkin spaces Ḟ α,q p (R m ). Our work requires more complicated analysis associated with the underlying geometry generated by the multi-parameter structures of the composition of two singular integral operators with different homogeneities. Therefore, it is more difficult to deal with than the duality result of the Triebel-Lizorkin spaces in the one-paramter settings. We note that for 0 < p ≤ 1, q = 2 and α = 0, Ḟ α,q p (R m ) is the Hardy space associated with the composition of two singular operators considered in Rev. Mat. Iberoam. 29 (2013), 1127-1157. Our work appears to be the first effort on duality for Triebel-Lizorkin spaces in the multi-parameter setting.