The two men sitting on the stools at the Wild West Show were Frank James (Jesse's brother) and Cole Younger. Both were on the 1876 Northfield, Minnesota bank raid in which bank clerk Tom Heywood was killed. Younger was convicted for it but Mattie, and others, believed that Frank James did it.
Mattie arrived at the Wild West Show because Rooster had sent her an invitation. He knew he was dying. When she approached the two old men, Cole Younger stood up, took his hat off, and explained that Rooster had died three days earlier when the show was at another town. He finished by saying, "We had some lively times," thereby implying that Rooster had some history as an outlaw. Frank James remained seated with his hat on and said nothing.
When she departed she thanked Cole for sharing the information and remarked to Frank, "Keep your seat, trash."