How do you mean 'separate setup'? you mean a second hifi? If this is the case, you will only be able to stream to one hifi at once I think.
If you have two airports, you should consider creating one wireless network with them. Set one airport up as a mian basestation, and then have the second one join this network as a remote basestation - it extends the network and is a lot neater than having to keep switch between networks if you have two separate ones.