If I have two Vanguard Knights equally forward, do they cancel each other out?
Yes. They only get the benefit if they are the true vanguard, with a single minion being the farthest forward.
How do I know if they're the 'farthest forward?'
The row of squares closest to you is 1, then the next row is 2, then 3, then 4. The higher the number, the farther forward it is.
