If you continue to look into the abyss then when does the abyss take its turn to look into you? Do you stand there forever; or, until you starve to death or just fall asleep? By definition, at some point, you stop gazing. At this juncture does the abyss decide to commence his/ its vigil of yourself?
A conditional (simple conditions) sentence using future-perfect; and, when this observation thing has been completed (looking into the abyss), in the future; because you finally get bored or hungry the consequence (the abyss will start to observe you) requires a simple future-tense. (In such sentences the future-future combination tends to be reserved for threats:
"si huc venies, te interficiam" = "If you come here, I will kill you" (Nicholas Oulton: "So You Really Want to Learn Latin" III, p.80).
It could be argued that you find the abyss intimidating; but, given such circumstances, you might not remain long enough for it to begin to observe you.)
What if the observations are simultaneous? As you've mentioned--present indicative--for an accepted state-of-affairs--Allen & Greenough (section 465, p.293): "(the present tense) as indefinite, referring to no particular time, but denoting a general truth":
"obsequium amicos, veritas odium parit" = "Flattery gains friends, truth hatred." (Ter. And. 68).
The warning: "When you look long into the abyss, the abyss also looks into you." may be construed as "a general truth", qualifying it for the present-present tense-combination.