I think the most important thing is whose pay-roll you are on. If you are on Canadian pay-roll and was on a short term assignment where you have your per-diem/all expenses covered in the US - then you pay taxes only in Canada and none in the US. Make sure you have a permanent address in Canada, obviously you should have one as you would pay taxes in Canada. It all depends what your short term assignment means ??? If you are on US pay roll during this period, then it will create problems even if you pay taxes both in Canada and US