They are two mysterious inhabitants of Night Vale, a man who is not tall and a man who is not short. They are known to be enforcers for the secret organization that steals buildings from the underground city, packs them into crates (some of which tick and some of which are warm to the touch), and ships them through the Sand Wastes to warehouses for unknown reasons. However, it is not even known if they are always the same two men; when the man who is not short ceased to exist, he was immediately replaced by a new man who was also not short.
They are obedient, fearsome enforcers for the mysterious organization that has been moving crates containing tiny buildings in the Sand Wastes for unknown reasons, driving around Night Vale in a black sedan with tinted windows and hunting down anyone who stands in their organization's way with ruthless efficiency. When You, a pawn in the same conspiracy, stole one of the crates, it was the man who was not tall and the man who was not short who ultimately tracked You down in the Sand Wastes.
On April 15, 2014, after waiting for a man at his home, blindfolding him, and abducting him into their black sedan, the man who was not tall and the man who was not short met their supervisor at a warehouse in Night Vale's industrial part of town. She was angry about some recent, unspecified setback to the operation, insisting repeatedly that "someone must be to blame" and gesturing to a blindfolded man they had abducted. The man who was not tall understood this to actually be a veiled instruction to kill the man who was not short in the desert, a grim order which the man who was not tall appears to carry out; the man who was not short's existence ended when the man who was not tall moved his knife. After which he sadly echoes his supervisor's words, saying, "Someone must be to blame." The man who was not short experienced a vision of the the Dark Planet immediately before his death, feeling as though it was so close that he could reach out and touch it. The blindfolded man, whom the man who was not short had wrongly presumed to be their intended victim, was then revealed to be his replacement instead, a new man who was also not short. You experienced a very similar vision just before Night Vale Community Radio host Cecil Palmer abruptly stopped telling Your story, which, given that the man who was not short had been holding a knife to Your throat at that moment, suggests that You were probably murdered in the Sand Wastes immediately following Your vision, as well.
A man who was not tall and a man who was not short were also present when the submarine from Nulogorsk was breached by the Sheriff's Secret Police on February 1, 2014; this is the only action they are known to have performed that has no apparent connection to the mysterious crates in the desert. The man who was not tall carried away a wrapped parcel, while the man who was not short carried away a thick book.
Some years later They were seen escorting Thomas Charles Fleming out of the Moonlite All-Nite Diner after he tried to warn other people about stone crops. They apparently shot him. Shortly after this, Their truck was surrounded by angry Night Vale citizens and They were arrested by Sheriff Sam for murdering Fleming. After it was revealed that inside the plants They were transporting were people They murdered, They were released by Sheriff Sam and ordered to remove the plants. They left in their truck, taking the plants and the dead people away. 
When their story was reported by Cecil Palmer on April 15, 2014, the man who was not short was talkative, prone to distraction, and generally incompetent, whereas the man who was not tall was taciturn, focused, and perceptive.
They seem to have some sway with the Sheriff's Secret Police, as they were among the first to enter the submarine from Nulogorsk after the Secret Police breached it, and were allowed to leave with two mysterious objects. The Sheriff's Secret Police and A Vague, Yet Menacing, Government Agency appear to both be involved in the operation, as they once released a joint statement trying to draw public attention away from the crates in the desert.
They are occasionally narrated by the man on the radio. They generally don't mind; there is no need to hide what they're doing, not in Night Vale. However, the man who is not tall glares at the radio when he lies to his partner and is contradicted by the man on the radio.