One problem you will have is that sperm from the 20th could still be alive and well in your reproductive tract on the 24th. Unless there is something an ultrasound demonstrates that differs from the assessment that you are 6 weeks 2 days today (January 23), you won't be able to tell which guy is the dad without a DNA test.
Hi, how did you get the 6 weeks 2 days figure, was it from an ultrasound?