Multiple Send Connectors: same address space, different smart host and source servers

There's a bit of black magic that goes on under the hood with internal message routing, and I don't believe you would get the results you are looking for (ie users still connecting to 2013 use the old mail scanning server, while test PC's connecting to the new 2019 server use the new mail scanning server for outgoing mail). If you want to be guarantee that only certain users use a send connector, I'd look at using transport rules to send mail from certain users to a specific connector.