problem solved. my cab only had coin sw 1 wired up. coin sw 1 adds credits for player 1, and coin sw 2 adds credits for player 2. so i added coin sw 2 to the jamma harness and everything works.
Does Double Dragon allow you to start a two player game from multiple coinis in one slot? I am not familiar with how the game coins up.
The other thing I can think of is are you using a Jamma extender harness? I had a similar problem with some Konami boards and found it was due to the wiring on my Jamma extender Harness. Make sure that both coin mech wires aren't going to the same side of the harness.
Use this link to check the Jamma pinout: http://www.jammaboards.com/jcenter_jamma_pinout.html
This is another good site to reference. It has a Neo Geo / Jamma pinout comparison: http://www.hardmvs.com/