I'm planning to go back to my country next year (probably by Feb) and come back again to the US. I want to bring my wife with me back to the US. So for me it's round trip but for her it's single trip. Is there any website that allow me to book round trip for me and single trip for my wife? The problem is that when I use Kayak or Orbitz, I can't find these options, so I have to book for myself and then take the flight number and search for it again!
If you want to book this itinerary online, you will have to do exactly what you mention: book the roundtrip for yourself, and then separately book the one-way ticket for your wife. Make sure both itineraries are available, and then book one and then the other. Many airlines (such as Continental) now allow you to cancel your itinerary within 24 hours for a full refund, which mitigates the risk of the second itinerary not being available after you book the first one.
You could also simply call the airline and make the reservation directly over the phone. In this case they would be happy to book both itineraries simultaneously. A travel agent could also do it for you. This is always the easiest way to book any kind of non-standard or unusual itinerary.
You might find that the one-way ticket is ridiculously expensive. On many airlines, one-way tickets actually cost more than round-trip tickets. If this is the case, it may be easiest to simply book two round-trip tickets together, and simply not use one of the return trips. Some airlines now have good deals on one-way tickets obtained using reward miles.