It seems like there are one or two convoluted methods to manually reset a given contact's phone number to stop sending iMessages to them, involving switching the iMessages setting on and off and rebooting and resetting the phones while also deleting the contact and manually adding it again on one of the iPhones. That way seems pretty crazy, and I hope Apple figures out a better way to resolve this issue.
Meanwhile, if you send a message to a non-iMessages user, and the message shows a red exclamation point (showing it didn't send), you can tap on the exclamation point and choose "Send as Text Message". Do this two more times to the same contact, and it seems the Messages app finally catches on that you only want to send text messages to this particular contact.