If the nipple is under 24" long than you have exception #3 in 310-15. If it is aloud in NEC it dose not allways mean it is best solution. Maybe use a seperate nipple. Or better yet can you figure another way into the right panel. I don't know how many wires your talking about. It might make the on panel a rats nest. That could lead to confusion and someone making a mistake. Out of phase networks, nuts in wrong panel, mismarked breakers, or pushing fishtape into wrong panel. You or the next guy.
Tom
