Hello Wayne,
I had composed a response yesterday but I must have forgotten to submit it.
I have tested giving the Pi a fixed (static) second IP address with out any noticeable issues. I was fortunate enough to have already set up the wifi connection for the Pi so I could simply SSH into the Pi using PUTTY.
Once the SSH connection was made and after entering Username and Password, I exited the Portsdown menu (ESC), this gets you back to the command prompt where I entered
sudo nano /etc/network/interfaces
This opened the editor, I moved the cursor down to after the code that looks similar to
auto lo
iface lo inet loopback
iface eth0 inet dhcp
I then added the following text to the code
auto eth0:1
iface eth0:1 inet static
address 192.168.2.2
netmask 255.255.255.0
Pressed CTRL + X, Y to save, Enter to keep same name
Once back at the command prompt I typed
sudo reboot
and then closed my SSH window on my PC
I connected a XP laptop to the Pi via a cross over cable and then set up the network interface for DHCP and fail over to fixed IP but found there was a delay in windows trying to find the DHCP server (Which is not a big issue but I wanted to get it going) so I resorted to setting a fixed IP of 192.168.2.1 / 255.255.255.0 in the laptop. Once again I was able to use PUTTY to connect to the Pi at the new address of 192.168.2.2
I also found that either the Pi or the laptop is able to do network cable sensing and switched the pairing to suit so I did not need a cross over cable in the end.
It looks like I was wrong thinking of having a DHCP and a second IP address, it is not possible (tested in XP), it is possible to have multiple IP addresses assigned to the one Network card, but you have to set them all yourself ie no DHCP issued IP addresses
So for the Fail over to fixed IP address you click on the "Alternate Configuration" tab and enter the fixed IP address you want your PC to have after it has failed to find a DHCP Server on that network connection as can be seen in the web page below.
I found these instructions for windows 10, which is the same as windows XP just have to navigate to it slightly differently, to set up a second IP on your network card.
https://routerunlock.com/set-multiple-i ... -computer/
Regards,
Bevan
PS We have in the past made a FM 23cm contact into two of the repeaters down your way, Summerton and Mount Terrible.