refowarding the radius request when authentication fails

Hello, in the authentication step, i try several authentication against otp server, but if all are failed if the user is not know, i would like to re forward the radius request to another radius server. except the suffix mechanism in authorize section that i know a little bit, i don't see how to do that easily in the authentication section. The only solution i see, but not very nice, is to build a radius client in perl (define in the authentication function (rlm_perl)). If someone has an idea ??? Regards, Laurent