I did something like this with my DrayTek devices. However, the DrayTek devices also ended up broadcasting PPPoE connection attempts for some reason so I had to isolate the 'second cable' on each of them using a VLAN tag on my EdgeSwitch. Now they can no longer see each other, just the FireBrick, EdgeSwitch and my virtual machine which monitors and records DSL statistics from both of the DrayTek devices.