Code.gs

var calsToSynchronize = [
  "address-one@gmail.com",
  "address-two@gmail.com",
];

var BUSY = "Busy",
  COPIED_FROM_CALENDAR = "copied-from-calendar",
  COPIED_FROM_EVENT = "copied-from-event",
  TIME_HORIZON = 365 / 2; // scan for 6 months

function synchronize() {

  var today = new Date();
  today.setHours(0, 0, 0, 0);

  // Scan the year in one-day chunks because...
  // it avoids collapsing of recurring events
  for (var day = 0; day < TIME_HORIZON; day++) {

    var eventsByCalId = {};

    calsToSynchronize.forEach(function(sourceCalId) { // no [].map available here :(
      var sourceCal = CalendarApp.getCalendarById(sourceCalId);
      var oneDay = new Date(today.getTime() + (day * 24 * 60 * 60 * 1000));
      var nextDay = new Date(today.getTime() + ((day + 1) * 24 * 60 * 60 * 1000));
      eventsByCalId[sourceCalId] = sourceCal.getEvents(oneDay, nextDay);
    });

    calsToSynchronize.forEach(function(sourceCalId) { // no [].map available here :(

      eventsByCalId[sourceCalId].forEach(function(sourceEvent) {

        if (sourceEvent.getTag(COPIED_FROM_CALENDAR) || sourceEvent.getTitle() === BUSY) {
          
          // we created this from an existing event. See if it still exists!
          var stillExists = false;
          var possibleMatches = eventsByCalId[sourceEvent.getTag(COPIED_FROM_CALENDAR)];
          possibleMatches && possibleMatches.forEach(function(possibleMatch) {
            if (possibleMatch.getId() === sourceEvent.getTag(COPIED_FROM_EVENT)) {
              stillExists = true;
            }
          });

          if (!stillExists) {
            sourceEvent.deleteEvent();
          }

          return;
        }

        calsToSynchronize.forEach(function(targetCalId) {

          if (targetCalId === sourceCalId) { // no [].filter available here :(
            return;
          }

          var alreadyExists = false;
          eventsByCalId[targetCalId].forEach(function(possibleMatch) {
            if (possibleMatch.getTag(COPIED_FROM_CALENDAR) === sourceCalId &&
              possibleMatch.getTag(COPIED_FROM_EVENT) === sourceEvent.getId()) {
              alreadyExists = true;
            }
          });

          if (alreadyExists) {
            return;
          }

          CalendarApp.getCalendarById(targetCalId)
            .createEvent(BUSY, sourceEvent.getStartTime(), sourceEvent.getEndTime())
            .setDescription("Busy status propagated from " + sourceCalId)
            .setTag(COPIED_FROM_CALENDAR, sourceCalId)
            .setTag(COPIED_FROM_EVENT, sourceEvent.getId());

        })
      })
    });
  }
}


function debugListCalendars() {
  var cals = CalendarApp.getAllCalendars();
  Logger.log("You have " + cals.length + " cals");
  var c = cals[0];
  cals.forEach(function(c) {
    Logger.log(c.getId() + ": Name=" + c.getName() + "; Description: " + c.getDescription());
  });
}