But that's what happened last week, when the Spartans gave Riley -- a redshirt freshman who had been backing up Max at middle linebacker -- a shot at tailback. Suddenly, the brothers were pitted against one another and knocking heads on running plays.
"We're both very competitive," Riley told ESPN.com. "It's definitely something to see."
Putting the younger Bullough in the backfield was a surprising sight. Riley said he hasn't played running back since early in his freshman year of high school, though he was a prep quarterback who was used to having the ball in his hands. He also emulated Iowa's Mark Weisman on the scout team last year. Most assumed he'd follow the same path as Max, who is an All-Big Ten linebacker and future pro at the position.
But Michigan State head coach Mark Dantonio has auditioned several players at tailback this spring as he seeks a replacement for Le'Veon Bell. And Riley scored two touchdowns -- both on 1-yard runs -- in last Saturday's scrimmage.
"It was a big adjustment at first, but my whole life I've played offense, so I was kind of used to it," he said. "I've got a lot to learn, but the guys on offense are helping me a lot. I'm going to play running back a little bit more this week, and hopefully some in the spring game."
Riley is big for a running back at 6-foot-2 and 232 pounds, but then again, so was Bell. Riley hasn't exactly been leaping over people like Bell so far in practice.
"Being 230 pounds does help out, especially going against our defense," he said. "They're a powerful defense. I'm not trying to juke anybody right now. I'm just trying to hit the hole as fast and as low as I can."
Not surprisingly, Riley views himself as a downhill power runner, but says he's got more speed than people might think, though his shake-and-bake moves are a little rusty.
Running backs are not a new thing in the family. His grandfather, Jim Morse, played halfback at Notre Dame in the '50s, and his uncle, Bobby Morse, was a Spartans fullback in the '80s. Riley said Bobby Morse will be at Saturday's spring game, and he plans to pick his brain about the running back position.
Michigan State is still using Riley at linebacker in practice, too, so he's learning both positions. Is his older brother jealous that he gets to play with the ball in his hands?
"He's a defensive guy through and through; always has been," Riley said of Max. "He was just as pumped as I was when I first told him about it."
Max is known as one of the smartest players around, and Riley said his brother's football IQ is so high that he can even give him tips on the offensive scheme. Even if Riley ends up on defense, he thinks this experience will help his own understanding of the game.
"I'm kind of learning some of the schemes and how offense work in general," he said. "Going back on defense, I can picture in my head the entire play, not just what the defense is doing. So that helps a ton when you're trying to play linebacker."
For now, he's a two-way player. Even if that means running into his brother on occasion in practice.