Two on 1 means 1 guide guides 2 hunters at the same time. Usually not an issue depending on the game and if the other person hunting with you is a friend rather than a stranger.
Usually 10% of hunt cost would be a starting point for a tip to a guide but really it is what you feel. If you have a lousy guide who doesn't work for you then tip less. If the guide goes all out and puts you on a trophy of a lifetime then maybe more.