It can work with just 2 but your more then likely going to end up with a dead fish 9 times out of 10. Generally the only way to get just a pair to work is to add multiple to the tank, let them all pair off and then remove all the other pairs ya don't want.
Another method I found that works is to house another large fish close enough for the pair to see. Generally this will switch their attention to this new "intruder" rather then infighting. Granted I would use that with a grain of salt as this may have worked for me but it may not work for everybody else.