The ability to run two instances of Dune with one automatically connecting to the RPC server of the other is something we want to add in the future, but most of our development time has been focused on package management these past few months.
For dune exec there is actually a specific option that would fit your use case better called --no-build. You will need to build the executable in the watch mode dune but you should be able to execute it without running into the lock separately. This is probably what you want to do. I am however unsure how this interacts with -w for exec.
@mudrz Could you create an issue in the Dune repo. I think it should be straight forward to support such a feature and it would be beneficial to other users too. We can flesh out a design in the issue.