What do you mean? Any of these cards I think is always is any cost combination of the 2 elements involved.
It is, but not the way Brontos proposed it. Normal Dual Cost cards cost X
and Y, while Hybrid(aka "Duality") cards cost X
or Y. The difference is that you could theorectically mono with a Hybrid card although it might not be as good as a mono card.
Darkness creatures lose Vampirism and gain +2/+0.
Light creatures gain -2/+0 and Vampirism.
This seems like a decent idea, considering that it works both for and against the Elements it associates with though. 4 Darkness/Light seems like a fair cost, given the mass-buffing nature.
Slightly offtopic:
I like the original border you came up with, ET. Could you make a
/
and an
/
one ?