[Exim] manualroute & hub question

We have two mail gateways gateway1 (with the lower number) and gateway2
(with the higher number) so that if gateway1 goes down gateway2 would
automatically take over.
Now for some internal machines we had the send_to_gateway as below:

send_to_gateway:
driver = manualroute
domains = ! +local_domains
route_list = * gateway1.wmin.ac.uk bydns
transport = external_smtp

How do I ensure that gateway2 gets a mention there as well in case
gateway2 fails?

Regards

Sujit
-----------------------------------
Sujit Choudhury
ISLS
University of Westminster
115 New Cavendish Street, London
E-Mail: S.Choudhury@???
Tel No: 020 - 7911 5000 Ext 3851