I believe you will be able to access the United Club at EWR but not at BOS. According to united.com, buried down in a footnote, "travel to Canada, the Caribbean, Central America, Guam and Mexico are considered international itineraries for the purposes of United Club access." Passengers in United Business are entitled to access but "may only access United Club locations at connecting airports and their final destination," which seems to me to exclude the originating airport.