The shell will connect to an existing running instance, but there is no way to specify which one.
There’s also no way to run a command on all running instances. Have you considered adding the command you want to be executed on all instances to your build command?
If you have a specific use case in mind that couldn’t be addressed with the approach I outlined above, I’d encourage you to submit a feature request at https://feedback.render.com/. It helps to include as much context as possible about your use case, the problem you’re looking to solve, and how you’re getting around it today to help us develop the best possible solution.
We rely heavily on customer feedback as a part of our planning and product roadmap process, so capturing interest on the feature request page is very helpful.