OK, I solved this problem - at least this worked for me. As stated, with more than one ttyUSB device, there is no way to accurately predict which device will be assigned to ttyUSB0, ttyUSB1, etc. So I edited my /etc/udev/rules.d/99-input.rules file and added the following:'
SUBSYSTEM=="tty", ATTRS{idVendor}=="10c4", ATTRS{idProduct}=="ea60", SYMLINK+="zwave"
SUBSYSTEM=="tty", ATTRS{idVendor}=="0557", ATTRS{idProduct}=="2008", SYMLINK+="x10"
...then I stopped HG:
sudo /etc/init.d/homegenie stop
...your vendor and product codes will be different - google the 99-input.rules file for how to figure out these values. Then I edited my /usr/local/bin/homegenie/systemconfig.xml file as follows:
<Interface Domain="HomeAutomation.X10" IsEnabled="false">
<Options>
<Option Name="Port" Value="/dev/x10" />
<Option Name="HouseCodes" Value="C" />
</Options>
</Interface>
<Interface Domain="HomeAutomation.ZWave" IsEnabled="false">
<Options>
<Option Name="Port" Value="/dev/zwave" />
</Options>
</Interface>
...and restarted HG:
sudo /etc/init.d/homegenie start
This seems to work through restarts and reboots. The port does not show in the dropdown on the settings screen, but it seems to persist. Hope this helps someone.