This is a broadcast packet issue.
Bonjour (and a number of similar broadcast protocols) operate by sending an informational packet to 'everybody.' When your router (Airport) - and the Mac - receive packets, they open them, observe the recipient, and forward it to the appropriate location. This is where things get tricky, because when a broadcast packet is sent, it's actually sent to nobody in particular. So when your router sees packets of this type, it generally only forwards it to the other computers in the subnet. By having two DHCP servers (by having the bridging Mac share its connection) on the same network, you are essentially forcing it into two subnets. If you tried to have the two routers on the same subnet, you would have two DHCP servers issuing IP addresses - generally said to be a bad thing. The real issue here is that DHCP and Bonjour both operate using broadcast packets.
So what's the best way around this?
You can modify your subnet mask (generally something like
255.255.255.0) to be more general and include the next smallest group of IP addresses - probably by switching to
255.255.0.0. You'll have to do this on all devices on the network. However, this simply reintroduces the issue of dual DHCP servers.
The best solution is to have all your devices pointing at the same DHCP server somehow. You can do this one of two ways:
By transparently bridging the ethernet ports on MacPro1 using 3rd-party software such as IPNetRouterX.
If you're not opposed to obtaining additional hardware, you might consider purchasing a switch to attach to your AirportExpress and then attach both Mac Pros to that switch.
I would do #2. #1 requires MacPro1 to be on, and therefore results in more power usage and more potential downtime.