The first $3500 you earn from employment is exempt from CPP. So if your first T4 had CPP contributions listed, you may have been due a refund of those amounts. Once you entered additional income that climbed over the $3500 mark, if the second T4 had no CPP, there would be CPP due on the total income over the $3500 mark.
Double check that both of your T4s have been entered properly.
