The second guy, based on your ultrasounds' due dates.
Sugar, you said you can't do a DNA test for the baby because the second guy might leave you. What are the circumstances in which he would leave? Obviously he is not going to learn he is not the dad, he will learn he is the dad. A lot of guys (especially unmarried ones) like to have that confirmation before they sign up for a lifetime of supporting a child. Are you concerned that if he thought you had sex with someone else he would leave despite being the father of your child? What is the relationship like, are you two going to be married? In other words, is it shaky or strong?