This paper presents an interactive and improvisational jam session, including human players and two robotic musicians. The project was developed in an effort to create novel and inspiring music through human-robot collaboration. The jam session incorporates Shimon, a newly-developed socially-interactive robotic marimba player, and Haile, a perceptual robotic percussionist developed in previous work. The paper gives an overview of the musical perception modules, adaptive improvisation modes and human-robot musical interaction models that were developed for the session. The paper also addresses the musical output that can be created from increased interconnections in an expanded multiple-robot multiplehuman ensemble, and suggests directions for future work.