Anyway, what happens when you bring up two interfaces depends on how you configure the interfaces, how you set up your routes, and how your network topology is.

Note that having (at least) two interfaces is very common -- an ethernet port and a loopback interface and you already have two.

As for a script that checks the status of the interface, why a script? What's wrong with ifconfig?

Hello,
On my system...

#!/usr/bin/perl
use strict;

my @if_lines = `ifconfig`;
foreach (@if_lines) {
    #you could remove interfaces with
    next if /^stf/; #this says if the line starts with stf, skip it.
    next unless (/up/i); #this says only print if the line has up (ign
+ore case). 
print;
}
[download]

This would just show up interfaces.
You get the idea. You could put the system command into an array.
Then filter in or out what you want to see...
You can play with this idea. (Good project for yourself.)
Good luck, Rob

based in the former, a little golf