Voldermort was insance, Harry is his target, so obviously, he never spare Harry.
And James was Voldermort's enemy, how can he spare James? If Snape begged Voldermort spare all the Potters, then Voldermort will suspect Snape and kill Snape too.
Snape had reason to begged Lily life: he loved Lily. And Voldermor believed that and agreed with him to spare her. So, what is the reason he can use to beg for James's life (even if he want)? He loved James too? Voldermort would have laugh into his face.