First login to the homepage like normal. The Homepage should always show your main signin name, like "Lonnie" or "TheMachine" (AKA Planet Nagy). If you click on "My Account" in the upper right-hand corner, you can change what your main foosworld.com profile says (the one with your pictures that comes up when you do a player search).
Now click on the "Forums" tab. On the Forums page look in the upper right-hand corner and you will see "Signed in as". Click on your sign in name. You will get a page with 7 tabs that lets you change your Signature & Bio, About info, Avatar, Email, Site Options, Passwords and Statistics, and Favorites on your secondary profile. Click on the "About" tab. Under "name" put whatever name you want to post under, then save changes (at the bottom). Now click on the "Site Options" tab and tick 'yes" for "enable display name", then once again save changes. Any posts in the archive as well as any future posts you make should reflect the name you entered on the "about" page (edit: the name change may not show up on every thread in the "started by" column, but should reflect the change in the thread itself in your posted box). To go back to using your main sign-in name in the forum, just tick "no" for "enable display name" and all your posts will revert back to the main sign-in name again.
As far as I can tell the homepage and the main foosworld profile will always show your main sign-in name no matter what, but the other pages will show you signed in as the name you entered on the "About" page and enabled on the "Site Options" page.