[exim] How to relay to more than one authenticated server?

I'd like to set up routers in exim.conf for relaying with TLS
authentication to two servers (which of course expect different
userids and passwords) for two different domain lists. From looking
at my existing config and [1] I think I want something like this:

# routers
#...
smtp_foo:
   driver      =  manualroute
   domains     =  +foo_domains
   route_list  =  *  foo.example.com
   transport   =  remote_smtp

smtp_bar:
   driver      =  manualroute
   domains     =  +bar_domains
   route_list  =  *  smtp.bar.invalid
   transport   =  remote_smtp
#...
#transports
remote_smtp
   #...
   hosts_require_auth  =  foo.example.com : smtp.bar.invalid
   hosts_require_tls   =  foo.example.com : smtp.bar.invalid
#...
#authenticators

login:
   driver       =  plaintext
   public_name  =  LOGIN
   client_send  =  ${if{AND_THIS_IS_WHERE_I'M_STUCK}}

Can anyone tell me what to put in that "if" to detect which router is
in use? Or do I have to extract the recipient domain and compare it
to the domain lists?

Thanks,
Adam


[1]
http://www.exim.org/mail-archives/exim-users/Week-of-Mon-20050103/msg00068.html