Your desired layout is possible with mojoPortal. If you take a look a this page, you'll see some example content and css that will show you how to acomplish it.
The key here is to use the "Custom Css Class" option on the module instances so you can position them how you like on the page. Also, you must put all of the instances in one "panel." I used the "Center" panel for this example. Each one of the module instances has a class of "cpanes" and another class of "cpaneX" where X is the numbered pane (pane 1 has a class of cpane1).
Keep in mind that if you are only concerned with HTML content, you shouldn't use this approach. In that case, you should follow Joe Audette's advice about the single instance of HTML content feature.