How do I load aspx webform from another aspx webform?

Ok, I have Main.htm page which contains:
<frameset rows="20%,80%">
<frame scrolling="no" name="top" src="ONE.aspx">
<frame scrolling="yes" name="bottom" src="MAIN.aspx">
</frameset>
and I have another webform TWO.aspx

So the top frame (one.aspx) contains action buttons and
bottom frame is for display purposes.

So when user clicks button on ONE.aspx I use:
Server.Transfer("TWO.aspx")

I would like TWO.aspx to replace MAIN.aspx on the bottom,
but instead TWO.aspx replaces ONE.aspx on top.

All my code is written in behind-the-form, and would like
to avoid HTML asp scripts if possible.

Thank you.