Pictures released by the group show Muellas keeping close by Hercules' side as they exercise in the shelter's yard.
The two dogs were surrendered together to Fort Wayne Animal Care & Control in Fort Wayne, Indiana, in late February because their owner could no longer take care of them. Workers initially separated them, but soon realized they were miserable apart.
"[Muellas] loses his confidence. He no longer wants to brave walks or roam a kennel when he is alone," Jodi Hamilton, the shelter's public relations director, told The Dodo. "Hercules, when separated, just laid, sad-faced, moping and whining. After one full day, he started pacing and whining even louder."