I have a Watchguard XTM 8 series firewall, and it is setup in mixed routing mode. I have a /24 public IP range and I want to pass one of those IPs to another interface on the firewall WITHOUT using NAT - because I want to connect a Cisco router to it to do VPN Clients for Macs, which won't work behind NAT. Is there anyway to do this? I cannot figure it out for the life of me.
As they have a separate bridge feature that you need to configure - and you can't configure it to bridge external interfaces - I'm not too surprised it doesn't know how to deal with overlapping subnets configured on two external interfaces.
If you have a switch on the external side of the Watchguard, you could connect the Cisco router to it and they would both be in the same /24. You would lose having it behind the Watchguard, but that doesn't sound like a lot of change in security.
Otherwise, I think you need to look at drop-in mode.
The Watchguard SSL VPN client is available for Mac, too.