Re-entry to US after 6 months in Canada

Hi
I'm an Aussi and my boyfriend is Swiss, were planning to travel through Canada for 6 months, west to east. On the way to Canada (from Australia) will stop over for 2 wks in Hawaii, so was planning to do the ESTA - visa waiver thing for that. We can both stay in Canada for 6 months visa free. The plan is to re enter the US over land around Niagra Falls and eventually fly out of NYC... so maybe 1 month in the USA.

My questions are:
can we re enter the USA on the same visa wavier?
if not, can we just re apply for another visa wavier.. or if because we will be within the north Americas for so long do we need some form of visa??

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