We do MFF on our airbus fleet between 330 and 320 family for around 10 years or so. The other big airline in the country uses MFF between A330 and A340, but not between A320 and bigger or between A380 and smaller.
We plan to do MFF on the 737 once the 787 arrives and currently a program for CCQ is being developed, we had approval for 737/757 and 737/767 MFF as well as an approved training scheme (three simulator sessions, some classroom training). Some management type pilots do fly both the airbus and the boeing fleet, however that is not done at the same time, they switch every half year or so between types.