Today was my youngest son's second run with Dad (yesterday was his first). He is 3 months old, a sturdy 20+ lbs and he tagged along in the Chariot (Dad's favorite push toy).
So far, he is taking to the Chariot just like his older brother. All wrapped in blankets and a hat to keep him warm, he drifts off to sleep after about 5 minutes. Then sleeps for the entire run until we stop at the house, at the end of the run.
I had considered trading up from the single Chariot to the Dual Chariot so I could push both boys at once. For now though I will stay with the single. I am not sure I want the two boys in the stroller until our new addition is a little older. By then, our first may be ready for a tag-along for bike rides.
As for runs, I think I will stick to pushing just one of the boys at a time. That is resistance enough, thank you very much.