I would keep it a duel setup and troubleshoot it as @WickedFog mentioned.
Traxxas wouldn't have made it a duel setup if the servos worked against each other.
I love any duel steering setups. Not saying single is a bad thing but on a bigger rig like the Erevo it's a awesome set up.
They don't work against each other, so I have to disagree with that one bud.
Something in there is binding but I stand firm on my suggestion.
It was designed that way for a reason.
It also voids the warranty.
Best of luck with it.
Welcome in @dshaf , I would guess your steering issue would be the fact you changed it from a double servo to a single set up.
I would put return it back to the original design.