Not sure if it is DCP, but you write it as a quadratic constraint:
$$\sum_k z_{k,i} z_{k,j} \ge 1$$
You can also linearize as follows:
\begin{align}
\sum_k x_{k,i,j} &\ge 1 \\
x_{k,i,j} &\le z_{k,i} \\
x_{k,i,j} &\le z_{k,j}
\end{align}