PTCGO should not have let you discard a DCE to pay for Dragon Blast's discard requirement.
That said, since the attack calls for the discard of a Water Energy and a Lightning Energy, and Double Dragon Energy satisfies that requirement, you should be able to simply discard the Special Energy card and be done with it.
That said, sometimes PTCGO has bugs that allow players to do things they shouldn't. See if you can replicate this error; if you can, submit it to PTCGO's bug report forum so it can be brought to their attention (if it hasn't already).