N.H. Rev. Stat. Ann. § 261:46

Notwithstanding RSA 261:45, the department shall register motor vehicles owned by individuals who are not residents of New Hampshire upon presentation of an affidavit by the applicant, on a form developed by the department, swearing that the vehicle is principally garaged or is regularly kept overnight in New Hampshire and that the applicant is the owner of at least one other vehicle which is registered and insured in the state of the applicant's residence.