I'd get my phone out and start recording, because two men with guns > one man with a gun, and I'd inform them that they're being recorded now. If I had a buddy, I'd get 'em to call 911 and report the situation, hopefully get some other officers to at least save the suspect's life. If I did feel the need to intervene, I'd approach their car from behind and grab the shotgun/rifle up front, because if I'm gonna be outnumbered, I may as well have the bigger gun.