Hi Lars,
All you need to do is to add another entry in your existing Encryption ACL
For example, if you had:
access-list 100 permit ip 192.40.200.0 0.0.0.255 192.168.77.0 0.0.0.255
to encrypt the traffic, then all you need to do is add:
access-list 100 permit ip 192.40.211.0 0.0.0.255 192.168.77.0 0.0.0.255
That's all on the Cisco side.
Make sure that Watchguard supports having multiple SA ( or multiple subnets ) going to a specific peer. I think there are many different vendors who only support one subnet per peer for the IPSec connections
Hope that helps
Jazib