I think you are focusing on the wrong issue. The multiple-submission issue may have long term consequences. It would be a problem even if you could get a complete refund of your costs for attending the first conference.
Before making any decisions, I suggest carefully reviewing all paperwork you or your advisor have submitted for each conference. You may, for example, have already assigned the copyright to the paper to one of the conferences. If so, you are better off withdrawing from the other conference. If you have assigned it to both, the situation is much more difficult.
Do not make up a reason for withdrawal. Conferences that might both accept the same paper are a small world. It is likely that the organizers of the first conference will at least scan the proceedings of the second one, and see your paper there.