well , I don't think that's going to be problem cos the US officers don't really check your passports according to my own experience . when you approach the US border you simply tell them that you are flagpoling and they will just take your passport , find a blank page and stamp on it saying flagpoling . I have to do this a lot with my boss s clients that need to get their study or work permits at the border . we usually go to Point Roberts in BC , and it has always been like this.
the point is , find a small and quiet border crossing , look normal and you will be fine . make sure you tell the US officers that you are flagpoling before handing them your passports. plus , since you are not really applying to enter the US , you are voluntarily withdrawing your application to enter the US during this process , you will be fine , that's the legal reason behind all these .