I've been wondering about this for a while. If you look at the equipment Turkish Airlines uses on some long haul flights you can see that they are both using Airbus and Boeing aircraft on the same flight.
As an example: on TK11 IST-JFK they are using anything from A330/340 to B777. It is not uncommon to see 5 days of Airbus service followed by a day of 777 and then Airbus again etc. It seems like they use whatever they have. There is no pattern and it is very random.
You can see the same approach on different flights like occasional B777's in ORD.
The same is true for Far eastern flights.
I'm wondering how they arrange the flight crews for such flights.
I couldn't think of many other examples for major airlines (except maybe airlines in the US) that use the same system, that's why I thought it was interesting.