Umm, actually the "normal" way to reproduce for a Beholder is to dream about themselves. If we talk about 2 Beholders, then I believe it would be through a "shared" dream – which is pretty available for a dream-interfering demon(like a succubus) as well.
Succubus: "So… where exactly are your sex organs?"
Bob: "…My *what* now?"
Succubus: "Oh dear…"
https://www.yafgc.net/comic/0004-just-ew/ suggests that beholders are capable of sexual reproduction somehow
Umm, actually the "normal" way to reproduce for a Beholder is to dream about themselves. If we talk about 2 Beholders, then I believe it would be through a "shared" dream – which is pretty available for a dream-interfering demon(like a succubus) as well.