On linux, one can use this
google-chrome-stable --proxy-server="socks://localhost:4444"
to start google chrome and tell it to use a local SOCKS server.
However this only works when chrome is not already running, otherwise it politely says
Opening in existing browser session.
and the proxy setting has no effect.
How can I use both a session with proxy and one without, at the same time? I don't want to use different "profiles" or "incognito".