This little hack permits to open more than one popup at time with leaflet.js

leaflet_multiple_popup.js

/***  Place this before Map initialization ***/
L.Map = L.Map.extend({
  openPopup: function(popup) {
    // this.closePopup();  // just comment this
    this._popup = popup;

    return this.addLayer(popup).fire('popupopen', {
      popup: this._popup
    });
  }
});