Sending message to contact with multiple (two) phone numbers – Message chooses wrong one regardless of what I prefer or put in.

I'm trying to send messages to a contact with two phone numbers and Message always sends it to the same number even when I specifically try to use the "other" one. I even also tried to manually type in the right number, and at first Message seem to understand what I want, but once I hit enter to send it off, Message automatically altered the number to the other one. I have now gone so far that I put the other number in an separate contact, but Message still bundles them up and ofcourse use the wrong number.

I don't have this problem on the iOS, only on the Mac (10.11).

/Alex