Hi Roy, thanks again for the advise. As you suggested, I tried using pure dark and zen dragon, first attempt I got a pure dragon, now second 54 hours. I had to see several times to believe it. Thank you so much. :)
Tinhinane... My son found out a new combination to get the Two Headed Dragon: Black Knight (level 20) + Kryptonite (level 20). We tested this combination twice with the same result. Hopefully it works for you as well.