I need to have two rows inside the main content area, is it possible?

mohsenasfia - You can achieve this from the Layout tab in studio. In the Layout tab, select the Layout which you want to edit, and edit it here, you can add one full row in the main content area. 

The XML of this would be something like:

<layout>
  <row id="header">
    <column id="common-header" width="24"/>
  </row>
  <row id="main">
   <column id="main-content-custom" width="24"/>
    <column id="main-content" width="18"/>
    <column id="empty-content" width="1"/>
    <column id="side-content" width="5"/>
  </row>
  <row id="footer">
    <column id="common-footer" width="24"/>
  </row>
</layout>

In the Canvas mode, it would appear as below:

Save your changes, and there you go. You can add as many as you want. Maximum width can be 24 Units, so your full row must have 24 units of width, as can be seen in the XML mode. I hope this helps.