What is your due date that they gave you from the ultrasound? Or did they just give you a due date from your last period?
I can't see anything in your information that suggests the guy from September 12 was the dad. Babies in utero change a lot in a very short time -- a doctor looking at the scan can readily tell the difference between a 7th week embryo and a 9th-week fetus. Your babies began in late August, according to your ultrasound. In your 9th week there is considered to be a couple of days' margin for error when using ultrasound measurements for this purpose, but not a two-week margin for error. In your imagined scenario in which the ultrasound was "off," it would have to be by a couple of weeks, and to a doctor trained in reading ultrasound scans, fetuses just aren't embryos. If you are super nervous, you might look up information on Google about what a baby looks like in the 9th week GA and up will come things like the crown-to-rump measurement and probably some photos. But a doctor is trained to assess what is shown in the ultrasound, you're not. Sometimes you just have to trust that a trained doctor is more accurate than your worries.
For a totally different reason, you should get DNA tests with the dad and babies once they are born. This is because unless he is your husband (in which case, the law will automatically assume he is the father), you might someday need legal proof he's their father. After the babies are born, doing the tests right at the hospital is often the easiest time to get a guy to do the swabs. (Go with him to the lab, and witness.) You need to protect your kids' rights to their father's support. Check with your doctor about how to set this up.