iFrame the main site on your landing domain!!
HTML Code:
<frameset cols='*' rows='*' FRAMEBORDER= 0 FRAMESPACING= 0 BORDER= 0 >
<frameset cols='*' rows='*' FRAMEBORDER= 0 FRAMESPACING= 0 BORDER= 0 >
<frame src='http://www.YourMainSite.com/' scrolling='yes'>
</frameset>
<frame src='http://www.YourLandingDomain.com/' scrolling='no'>
</frameset>
The last "frame src" maintains your landing domain in the address bar!