Thanks for your query.
If you are absolutely sure that you were ovulating bilaterally around conception, yes chances of twinning are high.
ALso, you are right, a very dark line , that too, before your missed period, means that the B - HCG
levels in your bloodstream are higher than normal.
One of the main reasons for a higher B - HCG is twinning, however to be sure, you need to consult a gynecologist and have an ultrasound
I hope you were not taking any injections or medications that could cause raised HCG.