My oldest two kids are 14 months apart, so DS started kindergarten one year after DD. They are both exceptionally gifted. We started subject acceleration for him in second grade, and grouped him with the highest group of third graders (which included DD). They got along and pushed each other.
At the end of that year, we began considering acceleration for several reasons. Before we even mentioned it to him, I talked to her about it and she was ecstatic about the possibility. So he skipped third grade.
They were in the same class last year in fourth, and again this year in fifth. They are best friends, incredibly goofy, and both so far beyond most of their peers academically that it's really nice that they have each other.
She's stronger in ELA. He's stronger in math (and has significant challenges with writing that make him 2E). There are moments of competition when one of them gets frustrated about "not being as smart," but those are honestly few and far between. And I think they would happen even in separate grades.
So, for us, I really think it was the right choice. Your kids sound similar, but I do have to admit that I would be more hesitant about the two-year difference than our one-year-difference. I think that might be harder for the older sibling emotionally. You know your kids best though.
