If you scroll down to the post "BBQ" you will see the boys have very similar bruises on their foreheads. No they didn't bonk heads. According to the story I got from Doug, Ethan wasn't paying attention and ran into the post of a handrail. Seth's stay is a bit more interesting. He decided to take a leap of the second from bottom stair into Uncle Jeff's arms. Only thing was Uncle Jeff wasn't ready for him and he face planted on the marble floor. All of this happened right as they were getting ready to take a family picture.
I guess as twins even their injuries have to look the same.