Faceless Void + Mars

Why this works

Picking both of these heroes on the same team often creates a problem of overlapping jobs. Both Chronosphere and Arena of Blood are massive, long-cooldown ultimates designed to control a teamfight. Using them together is usually redundant; Chronosphere freezes enemies, making the wall of Mars's Arena irrelevant and preventing him from using Spear of Mars to pinball targets. Faceless Void and Mars are trying to be the star of the same show, and their biggest spells don't combine well, with one often negating the key function of the other.

This forces the team into awkward situations. Either you use both ultimates for one fight, which is overkill and leaves you powerless for the next several minutes, or the heroes have to operate in separate parts of the map. A smart opponent will recognize this and force fights when your big spells are on cooldown or when one hero is caught without the other. Instead of creating a single powerful teamfight, you end up with two heroes who would rather have different teammates to enable their specific lockdown ultimates.