0.html

<frameset cols="50%,50%">
  <frame src="1.html" />
  <frame src="2.html" />
</frameset>

1.html

Hi, I am 1!<br/>
<a href="x0.html" target="megawindow">x 0</a><br/>
<a href="x1.html" target="megawindow">x 1</a><br/>
<a href="x2.html" target="megawindow">x 2</a>

2.html

<frameset rows="50%,50%">
  <frame src="3.html" name="megawindow"/>
  <frame src="4.html" />
</frameset>

3.html

Hi, I am 3!

4.html

Hi, I am 4!

iframe.html

Hi, I am 1!<br/>
<a href="x0.html" target="megawindow">x 0</a><br/>
<a href="x1.html" target="megawindow">x 1</a><br/>
<a href="x2.html" target="megawindow">x 2</a><br/>
<iframe name="megawindow" frameborder="no"></iframe>

mock.txt

--------------
|     |   3  |
|     |      |
|  1  |---2---
|     |      |
|     |   4  |
--------------

x0.html

Hi, I am X0!

x1.html

Hi, I am X1!

x2.html

Hi, I am X2!