Easy Setup What it sounds like you have
---------- ----------------------------
Internet Internet
| | |
Cisco Watchguard Cisco
| / |
192... 192...
10... |
Watchguard
|
10...
Go for the first design.
Have:
Cisco WAN: {Internet IP}
Cisco LAN: 192.168.111.1/24
Cisco LAN Secondary IP: 10.0.0.2/23 (a connection into the Watchguard subnet)
Watchguard WAN: {Spare Internet IP - assuming you have one}
Watchguard LAN: 10.0.0.1/23
Watchguard LAN Secondary IP: 192.168.111.2/24 (a connection into the Cisco subnet)
Now each device has an internet connection, a local connection on its main subnet, and a secondary IP giving it a leg into the other device's main subnet.
Then add a route on each device crossing the two networks over (Cisco -> Watchguard's secondary. Watchguard -> Cisco secondary), e.g.
Watchguard route: 192.168.111.0/24 via gateway 10.0.0.2
Cisco route: 10.0.0.0/23 via gateway 192.168.111.2
Your computers will send to their default gateway. If the traffic is for the other LAN subnet, it will hop between the two devices. Either device can send to the internet. Firewall rules are completely separate.
If you can't do this design because you don't have a spare public IP on your internet connection, then you will have more problems and need a more complicated setup.