By law (the US-Canada tax treaty) you have to pay
taxes where you live, so if you live in Québec you have to pay Canadian/Québec
taxes.
Because you're physically there when the work is done (rather than commuting to the US) then either your employer has to have a Canadian payroll and issue you a T4, or alternatively you register as self-employed or set up your own company and invoice them.
Because you're US citizens, you still have to file a joint 1040 every year, but you individually file Form 2555 (or 2555-EZ if your income is below the foreign income exclusion limit). Under US law only the first $87,600 is exempt from US income
taxes, so if you earn over that (each) you are subject to dual taxation on the amount over that limit. I haven't looked at the marginal rates in the US lately but I suspect you're looking at over $100,000 before you get hit with any significant US
taxes.
This is how Uncle Sam stops US citizens from moving to The Bahamas and avoiding US income
taxes.
Looking on the bright side, as US citizens you don't have to file all the annoying non-resident alien tax paperwork to report your US-source income.
Even if you physically did the work in the US you would still effectively pay the same amount of tax if your tax home is Québec, the difference would be that it would be subject to US withholding and you would claim a foreign tax credit for that amount. As
taxes in Québec are higher, you pay the difference there so effectively you end up paying the same amount either way you do it, the only difference is who you pay it to.
Steve.