So you want to join a second league in RDNation?
You will first need that second leagues "League Join Code". In the upper right hand corner of RDNation once logged in, you will see your email address. Hover over that email address with your mouse. Select the "Join League" link. Once there, enter in your league join code.
You will then be added to the second league. From now on, you will be able to move back and forth between leagues under the same menu. Hover over your email address in the top right. Select the second league you want to participate in. You will then stay within that league until you select the other league again from the same menu.