If you have two phone numbers assigned in Messages on a Mac, how do you select which to use for each conversation?

Very simple question with seemingly no answer other than "you can't." On an iPhone, when you have two phone numbers you can select which one to use to send a message (e.g.: "Personal" or "Work.") In Messages on a Mac, while you can RECEIVE messages to both numbers there doesn't seem to be any indicator for which number the message was sent to, nor is there a way to start a new conversation from a specific number.