The translation is slightly misleading. The question is really, "Can two walk together unless they have an appointment?" The word-picture is of two people who have agreed to meet at a certain place at a certain time. Of course, its application has been extended to other things in terms of continuing in a way of life.
In terms of keeping the Sabbath, it means, if one wants to be in God's presence, no other day will do. God has an appointment with His people to meet with Him at a special time, a set time. Moreover, it is different from other time'even as one's appointment with a professional (like a doctor, dentist, or lawyer) is different from another. In everyday life, we make agreements to meet with certain people at a certain time. Unless we make an appointment and both parties agree on it, there would be no meeting at any time.
It is the same way with God. He reveals a time that He wants to meet with us, and if we should choose to meet with Him at a different time, sorry! He will not be there. He has put His presence in the time He has made holy. Perhaps this is an oversimplification, but the point is valid. God has set a time in which'by appointment'we are to meet with Him. No other time is acceptable to meet with Him in congregation.