Hi, you can’t make someone else server admin. There is only one server admin, who can control the actual server, but you can make someone a GM which gives them full control over the game itself. The start/stop of the server is not something that you really ever need to do and the option is mostly there to give you control over your instance if you want to lock access to it so your players can’t access it at all.
The main thing that you would be missing is the ability to install or update modules. If you do have the Game Manager enabled, then it wouldn’t be possible to give that ability to someone else, but if you don’t have it enabled, then you can go to your game configuration, set an admin access key, and share that access key with your co-GM, that will let them return to setup and authenticate into Foundry’s setup page, which does offer the ability to install and update modules and systems from it directly (not as nice as the Bazaar, but it does the job).
If that functionality is not available at this time are there any plans to add something along those lines in the near future?
Maybe in the long term, but not in the near future, no. There would be ways today, to generate an API key which also gives control over starting/stopping the server or installing content, then the other user might do those operations using that API key and custom commands, but it is very technical and not at all user friendly (some info about it is here : How to control your games using custom API requests and reset your worlds periodically).
I hope that helps!