when placing the 2nd airport hold control and click to place,
then select 'build seperate station'
Unfortanlty you cant make them act as one airport
(no matter how much you increase station spread)
i like this cause you can name your terminals
to create a 3+ airport hub you need to create towns (in scarnio mode)
near the planned airport (unless you know a mod for 3+ airports per town)
You can also use this feature to "semi - cheat" online
by placing a 1v1 station in then city centre
then holding control place an out of town station and
select the name of the 1v1 city centre station
Openttd - you find out new features and mods every day

How did you group your aircarft to airline specific????