accouting over more than one servers

[Fretz Marco]

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