Wow, I think this is becoming the most frequently asked question on here. "I live in BC and have a job there, can I buy a house in Washington because it's cheaper and commute?"
Not easily because you only qualify for a B-2 in the absence of anything else. And that only allows you to stay in the US for up to six months a year.
Your situation is different because you have an I-20 by the sounds of it so you will be in the F-1 category or similar. This will allow you to stay in the US during the course of study, provided it's a reasonably full-time course. Your boyfriend might qualify for F-2, check the regs. Given that both of you work in BC then the EAD issues aren't relevant.
However once your course of study is over then you're back to B-2.
You would have to file two tax returns but this is fairly trivial, the only additional paperwork is a 1040NR, which you report your income on and Form 8840, which basically says you claim closer association to Canada so you don't pay anything to the IRS (or alternatively, Form 8843 which is used by students). Then you just file all the Canadian paperwork as per usual, with a US address on it.
But the thing is if you reside in Washington and have no address in BC, you have to get a Washington DL, and that creates a whole raft of other problems, car insurance, registration, problems with healthcare coverage, etc.
If you have TWO residences and claim BC as the primary one for legal purposes, then you only file a 1040NR and a 8843 and everything else pretty much stays the same. You just carry on using your Canadian address on your T1 and DL.
Having worked all this out recently for a close friend of mine whose husband is a US citizen, I came to the conclusion it did make more sense to live in Washington and commute, because the
taxes are way lower in Washington, no State income tax, no sales tax and they are healthy and the lower taxes mean they can afford the healthcare no problem.
But she can obtain permanent residency easily in the US, you can't.
Steve.