Bind data share traffic to fw1 - how?

This one puzzles me:

I have two MacPro (one model 3,1 - one 6,1) connected via FireWire=>Thunderbolt cable. Data traffic is smooth and a command line check confirms the firewire connection:

XPro:~ $ route get X8home.local

route to: 169.254.108.76

destination: 169.254.108.76

interface: fw1

mtu: 2030

However, after some idle time, when the new MacPro wakes up, the two machines connect differently

XPro:~ harald$ route get X8home.local

route to: x8home

destination: x8home

interface: en2

mtu: 1500

Now throughput is hampered by various streams running over WLAN and I have to turn off WiFi on one machine to force a reconnect over fw1. How can I force the two machines to stick to fw1?

- Harald -