Message reply on Mac goes to wrong person
At the top of the conversation of the Messages app the contact should be listed under the To: header. You can use the arrow by the name to select more details and change which number you are replying to.

As an alternative you could delete their work numbers, or put it in the notes section of the contact to prevent Messages from mixing them up.

It's best to delete the conversation and start a new one after making changes like these to make sure that messages is using the proper contact info.