$("#firstChart").D3Chart(d3FirstOptions);
 $("#secondChart").D3Chart(d3SecondOptions);

(function ($) {
    $.fn.D3Chart = function (extOpt) {
       //...
    function drawBars() {
      //...
      rect.on("mouseover", function (d) {
             var dpChannel = d3.select(this.parentNode).datum();
              //...
              });
       }
      //...
    })(jQuery);

.on("mouseover.firstchart", ...)
.on("mouseover.secondchart", ...)