accouting over more than one servers

hello there ive got a problem. we are using 2 radius servers to auth our xDSL users quering from a CISCO PPPoE Router. how can i protect a user to dial in from a secound xdsl line if first radius is down and CISCO AAA useses the 2nd radius server? and btw: does anybody know how to setup up AAA on a CISCO IOS to send periodically accounting infos to the radius server? thanks and kind regards marco fretz