Are you saying you want different pppoe servers to auth via different radius servers? This can be done already.
Add your pppoe-server name as the "Caller-ID" of the radius server, then that pppoe-server will use that server.
example
/interface pppoe-server server
add authentication=pap default-profile=default disabled=no interface=\
bridge-pppoe1 service-name=pppoe1
add authentication=pap default-profile=default disabled=no interface=\
bridge-pppoe2 service-name=pppoe2
/radius
add address=1.1.1.1 called-id=pppoe1 secret=RADIUSSECRET service=ppp timeout=3s
add address=2.2.2.2 called-id=pppoe2 secret=RADIUSSECRET service=ppp timeout=3s