One can define products on the Ktheory of graded C*algebras as in http://web.me.com/ndh2/math/Papers_files/Higson,%20Guentner%20%202004%20%20Group%20C*algebras%20and%20Ktheory.pdf on page 152, but how can we show that when $C*$algebra is unital and trivially graded, then the products agree with the product on the usual K_0 defines like $K_0(A)\otimes K_0(B)\rightarrow K_0(A\otimes B)$ by $[p]\otimes [q]=[p\otimes q]$. I think this is equivalent to show that the Fredholm index of $D_1\otimes 1+1\otimes D_2$ is the tensor product of the indices of $D_1$ and $D_2$.
The cupcap product of Kasparov, in bivariant $KK$theory, is defined for graded $C^*$algebras; for ungraded algebras, and when the first argument is $\mathbb{C}$, it restricts to the product you describe. See: Kasparov, G. G. Equivariant $KK$theory and the Novikov conjecture. Invent. Math. 91 (1988), no. 1, 147–201. 

