I don't know how I missed Peter's original question, as this is my third-great-grandfather, and a profile I created!
Jamie Nelson's answer feels right, though I wish there were a way to change that behavior. The senior Isaac Irwin-6746 shows up in my DNA Ancestors View. I am Irwin-2986.
Isaac Irwin-6746 and his wife Martha Unknown were the parents of both Isaac Irwin-6744 and Elizabeth Irwin-6756 Stewart. Isaac Irwin-6744's son, Edward Augustus Irwin-6742 married the granddaughter of Elizabeth Irwin-6756, so they are 1C1R to each other.