The current distribution does not support running more than one node on the same machine. When the concordium-node tool is run, while another node is already running, it will ask you if you want to stop the currently running node and start a new one.
A node can only run as one baker. A baker is by definition a member of the committee. The current committee can be obtained by querying the node via
$ concordium-client consensus show-parameters --include-bakers
A node can run as a baker or not, depending on how it is started, and whether the keys it starts with are registered in the committee.
Which baker the node runs as is determined by the baker keys that it starts with (these are in the file baker-credentials.json ). If a node is restarted it reloads the keys that exist at the time it starts.