Deejay - do you have an undergravel filter in your tank? If not do you have more than one filter where only one is cleaned at a time? The only other way you will be getting away with this unconventional practice is using water that doesn't contain chlorine and is around about the same temp as your tank. If none of these apply, I would suggest you touch some wood as often as you can 8)
And if your partners tank is getting nitrites too often it is time to get a better filter!