Ok, these are just ideas on how I would address this problem.
1. Login to each machine via unRAID GUI interface and check each machines ip addresses and record.
2. Set each machine to a static ip address.
3. Login to router and set those two address's as reserved
You can also verify the address of each machine by using console. ifconfig
The problem your having with your second machine I had the same issue when I had my Nest Thermostat and my server attempting to share the same ip address.
When I had two servers setup I had a bizarre problem as well using Tower. I ended up just addressing each machine as their IP and avoided using Tower, but changed each machine one to Tower and the other to XEON to avoid it, but make sure them IP's are conflicting.