So if two beholders were to face each other with their large central eyes, which emit an anti-magic field in the shape of a cone, would they both fall to the ground helpless?
Here's another question: If a beholder looks into a very large mirror, would it's own rays or anti magic field be bounced back at it? Does the field penetrate walls? Would it only be blocked by a mirror, but not reflect anything back?
If you're looking for an official answer, you might want to specify which edition.
If you're curious how different people rule, I'd say the eye rays are rays, and so are treated as rays. Since they're pew pew beams, they're not comparable to the gaze attack of a medusa, who turns anything to stone that meets her eyes, including her own self in a mirror. So no mirror reflection. The main eye also isn't a gaze attack in the mirror reflection sense, it's a field emitted by the eye. Since it's a field, I'd say it penetrates barriers.
And yes, beholders would negate each other. One of many reasons why they're solitary.