Hi, did you read the FAQ's at the top of this community? They address the question of how reliable it is to try to guess when your next ovulation is based on your last period. (Not in fact very reliable for most women.) https://www.medhelp.org/posts/DNA---Paternity/FAQs----Who-is-my-babys-father/show/3035989
They also talk about the time order for doing things to try to tell who the dad is, and would be a helpful list for you because you're nice and early.
To do the best you can with period dates, you count backward from when the next period was due (if they are regular) rather than forward from when the last one came. (Ovulation happens 14-15 days *before* the next period, not any certain number of days after the last period.) The 27- to 31-day cycle length you mentioned suggests your period might have come on January 11, 12, 13, 14 or 15. Counting back from that suggests ovulation any day between the 27th of December through January 2. This span of time unfortunately includes both men. (Though you had sex with the second guy on January 3, an egg can last 24-36 hours in your body waiting to be fertilized.)
As the FAQs suggest, the next step would be to get an early ultrasound, which you're going to do next week. I caution, though, that with the sex being only two days apart, even an early ultrasound might not help you sort this out. If sperm was in your body from the first guy (and it could have been), it would have been still alive in your reproductive tract at the same time as sperm from the second guy. But it is still worth a try to get an early ultrasound, and it sounds like your ultrasound will be in your sixth week, a good early date.
For the best information from your ultrasound, be sure to ask for the estimated due date of the baby based only the measurements of the baby, not on the first day of your last period. (Especially, don't just let them pat you on the head and look at a little cardboard wheel and just tell you a number of "weeks pregnant.") In fact, if you have not already told them when your last period was, tell them you can't remember for sure. That way, if they give you a due date, you can be confident that it will be only from the measurements and developmental markers of the baby, and not from them counting forward two weeks from your last period. If you have already told them when your last period was, tell them that your cycles are really irregular, and that you'd like the due date to be based only on the baby's measurements and not on when your period was.
The reason I don't think the first guy is ruled out is that his timing is good, and sometimes when a guy is barely able to keep an erection, it's because he has recently ejaculated (a wet dream, masturbation, or sex with someone else). If a man has had a relatively recent ejaculation, it leaves some sperm in his urethra, a nice, perfect-temperature and moist location that is not dangerous to sperm. The fluid from the Cowper's gland that lubricates the penis prior to sex ("pre-come") provides a very nice ride for any sperm waiting around in the urethra to go right into the lady's vagina or cervix. That's why "pulling out" does not work as a birth-control method, and why we get women writing in and saying they don't know how they got pregnant because the guy pulled out before ejaculating. The spermies took a ride on the pre-come bus and went right to the place whether he came with them or not.
Anyway, write back when you get an estimated due date from your ultrasound. Don't let them try to send you away with just a "weeks pregnant" count. (Especially since doctors often base those on when the woman's last period was, they aren't very helpful for what you need to know.) You can use the estimated due date to count back 266 days to the estimated conception date (on a calendar, or there are online calculators for this). At the best, maybe it will indicate that you conceived late enough to suggest the first guy is less likely. But I'm afraid it probably won't rule either guy out.
Given all of the above, you should also consider whether to do a prenatal DNA test from Ravgen or the DDC. They are expensive (ten times more costly than doing a DNA test after the baby comes), but at least then you will know. If you do it, please do it with both guys. One man's positive will rule out the other man's negative and vice versa, and you won't have to worry about the test being somehow mistaken.
Good luck, write back!