nginx on a separate server as a load balancer (optionally caching) is an entirely separate use case, and a reasonable one. But there is no performance benefit to nginx+Apache over just using nginx appropriately by itself (and for the vast majority of users, Apache is also sufficient…the performance difference between nginx and Apache in most users workloads is lost in the noise compared to the performance of the app and database).
But, a separate server as a load balancer is not a Virtualmin question. Cloudmin might get that kind of feature, though.