Garfield, debating whether to eat a donut, was confronted by a shoulder angel and shoulder devil in the shape of donuts (respectively marked by a halo and a devil tail). He ate the donut, then ate both angels. Appeared earlier here.
In a Pearls Before Swine Sunday strip, we see all four main characters (Rat, Pig, Goat and Zebra) go through this... except Rat, who has two shoulder devils.
Another Sunday strip has Zebra talking with one of the crocodiles, and explaining that he'd never be able to go through with eating Zebra because his conscience wouldn't let him. When the croc asks, "What is dis conshuss?" his Good Angel appears... and the croc pops it in his mouth and eats it.
The dailies also show tiny angels and devils fighting for Rat's soul. The angels give up without a fight.
In one Tina's Groove strip, Tina is confronted by what she assumes are her shoulder angel and devil, representing her conscience and temptation. They inform her that they're actually the team that warns her of embarrassing dandruff, and are there to recommend a good shampoo.
In an episode of Arlo and Janis, Arlo is visited by two copies of himself, both identical. One tells him he should be more ambitious, out to change the world. The other says that the world needs more honest, contented, hardworking family men and he's doing fine as he is. He asks, "Shouldn't one of you have a halo and the other horns?" They both shrug blankly.
This was spoofed in this◊ Dogs Of C Kennel strip, where Will sees that the kennel handler's wallet is exposed. Will's bad side appears, telling Will to do it, before Will lampshades about there not being a good angel, before his good side appears afterwards... and steals the handler's wallet then disappears, to then Will's bad side remarks, "Wow... you're one evil soul."