ive done some numbers and using your scan dating i estimate you concieved on the 3rd august it may not be spot on to the day but it will be within days of this date.Which one did you sleep with on or around this date (By around i mean a few days before or after 3/08/08 not weeks)this will give you a rough idea however if you slept with both too close together the you need a dna test it will be your best option in this situation as you need to be able to tell your son/daughter who they came from and im positive it will stop you feeling unnesasery guilt too this wasnt your fault but you need to do your best to fix this problem.I wish you the best of luck with this and hope i have helped you