Personally, I think the upgraded should not restrict the effect to one player - but I'm rather a fan of cards like Nightfall that can benefit both players. (In fact, 3 of 4 cards I've invented have this property!)
If I have 5 and 5 and I play a creature that costs 6 , what will happen? Will I either
A)lose 5 and 1
B)Lose 3 and 3 , or
C)Lose a random amount of both until it =6?
The only situation where this would matter is if the card gets destroyed or (upped only) stolen. PA removes the question entirely.
IF this card gets implemented, the elements should be placed directly opposite each other in the game window. Right now we have Time next to Light, Darkness next to Water, etc. - it would be a lot less confusing with matched pairs.
I'm not sure about this being OP in Rainbows. It takes a minute to set up, and a lot of rainbows rely on cards from both in a set of opposites - Firestorm/Permafrost, Druid/Oty, Steal/Sundial. Without a really good pillar start, the cost will cancel out a lot of the setup-speed benefit gained. In a rainbow vs mono/duo match, mono/duo will have a speed advantage as always, I don't see this getting around that. Rainbow vs Rainbow matches are one reason I'm in favor of having both versions of the card affect both players - otherwise a ton of rainbow decks would automatically have to include this for the slight eventual advantage it grants.