This fusion was not used with the "real" echo, but one created from the Eternal Mangekyou Kagegan with the help of Seireitou experience fighting Echo.
Seireitou used this form in a fight with Jake C. Tazuna. He noticed that his own power wasnt enough to take down Jake. At last, he remembers to how even though he was "stronger" then Echo, echo's genious still got the best of him. He thinks with echo's genious and Seireitou's raw power, this fusion could very well be one of the strongest ever.