It could be either. You WILL need a DNA test to identify paternity.
You don't conceive immediately. It can take several days AND sperm can survive for about 5 days.
You had sex on day 11 of your cycle (Jan 1). You then had sex on day 13 of your cycle (Jan 13). Normally you will ovulate between days 11 and 16 of your cycle.
Either man could be the father.