There's no easy or surefire answer to that question. It depends on the ages of the birds when they are introduced (the younger the better), the size of the pen (the larger the better), and the birds's individual personalities. It would be a trial and error experiment.
I keep both breeds, aged 3-6 years old, but keep them separate, . Rheas are lying, so could interfere in their laying pattern. Same with Emus, when they start laying again in a few months. Emus all in pairs, for same reason.
Calla