Use one config with two entrypoint?
It is better to use different configs with a shared part. For the server, for example, it makes no sense to pull in a bundle dependencies from node_modules. Variables can have different values, and so on.
Would it be possible to tie the hot-reload?
How to configure webpack to Express backend and frontend?
It all depends on the tasks before you and a set of tools that you want to use. Examples can be see in numerous articles on this subject and on Github.