No one can be married to more than one person at any given time. The fact that INS or any oher governmental agencies did not know about his two marriages does not change anything. If and when they find out, he can get arrested and deported. The primarly reason why INS does not know about it is because it is almost impossible to do a background check to determine whether someone is married more than once. However, if determined to find out, INS can find out about the two marriages, although they have no reason to search for the two marriages.