I have a number of questions about this situation.
Is shaking hands that important? Important enough to flag? I don't think so, but I can be convinced otherwise. If it is, it should be important enough to flag BOTH teams, since neither team shook hands. And then, if it is important enough to flag once, if they STILL don't shake hands, don't you then HAVE to flag them a second time? A second unsportsmanlike foul carries and ejection. Do you really want to disqualify the player(s) before the game even started for... being rude?
Do you really want to defend that?
Sending those players back and asking for new ones? Why, so the same episode can be replayed?
Making a big deal out of this, to me, just serves to escalate the situation that we are called upon to calm down. Finish the coin toss, send them to their sidelines with a warning that sportsmanship is expected, and their childishness has removed any benefit of the doubt that might have been given previously.
I would address the issue with the coaches, but I would not bring them to midfield to do so. As the R, I would take my U and the wings working that sideline and tell each head coach about the display of immaturity by the players designated as captains. He honestly may not know, as I assume coaches are somewhat busy during this time. I would tell him that we will call the game fairly, as expected, and while I expect his assistance, whether we get it or not, we will maintain control of the contest. As was ALREADY mentioned in our previous discussion, we expect all players and coaches to conduct themselves in a sportsmanlike manner, and anything less than that will be penalized.
At that point I would issue each team a sideline warning, and move on.