If your application is not just that big and you are just starting up. Its good to keep them on single server. Its will reduce overhead of managing two separate applications, and deploying separate applications.
If server side rendering is required for React js and you are expecting a heavy load on servers (High Number of Visitors) keep them on different servers. Rendering templates on server consumes enough of processing power.
If you are seasoned programmer and loves clean and clear architecture then using two docker containers is no doubt a good idea.