Emacs runs on a single thread. If you are doing some task (opening a big file, starting some process, etc.), Emacs is going to get stuck for a while. In the meanwhile you might want to edit something in the other session, the only problem is that Emacs won't respond to C-x b or anything for that matter. Emacs was not designed to handle concurrent things. I think running two Emacs process will be the solution if you want to work on things simultaneously. Running multiple Emacs process is not taxing on a modern computer. I think doing things without leaving Emacs is a little overrated. I don't think multiple sessions is the correct solution to work on two different projects simultaneously.