;(function$) {
    $.plugin2 = function(el, options) {
        ...
        ...
        var init = function() {
        plugin.settings = $.extend({}, defaults, options);
        plugin.el = el;

        var init = function() {
            ...plugin1Instance.settings.variable... <--------
        }

        init();
    };
});

var plugins = [];

function startOnLoad() {
   // Start init() of every 'registered; plugin
   for (var i = 0; plugins.length - 1; i++) {
      plugins[i]();
   }

   // Ok, all plugins initialized, can access each other.
   // Ex: plugin1Instance.settings.variable and etc.
}

plugins.push(Plugin1.init);

var windowOnLoad = function() {
    // your plugins should be loaded at the moment
}
if (window.addEventListener) {
    window.addEventListener('load', windowOnLoad, false);
} else if (window.attachEvent) {
    window.attachEvent('onload', windowOnLoad);
}

var totalPlugins = 2;
var numberOfLoadedPlugins = 0;
var pluginLoaded = function() {
    numberOfLoadedPlugins += 1;
    if(numberOfLoadedPlugins === totalPlugins) {
        // it's done. your plugins are loaded
        // proceed with the other logic
    }
}

;(function$) {
    $.plugin1 = function(el, options) {
        // ...
    };
    pluginLoaded();
});
;(function$) {
    $.plugin2 = function(el, options) {
        // ...
    };
    pluginLoaded();
});