fixing problem with instance
 

hi guys. i'm working on a project for school, we were supposed to build a robot by opted for a dinosaur because it seemed more challenging, and fun. and it is! one problem though, ive created an instance of parented objects and when im trying to weld the two objects on the left side, the ones on the right side wont respond. just from looking around i think it might have something to do with normals? i have no idea how to fix it. heres a screenshot, and the file too. the problem is at the base and top of the neck/head intersection, the sides worked out fine...

http://www.mediafire.com/?dmijed1wvj

http://img155.imageshack.us/img155/1356/ss1copyev2.jpg

depending on which side i select, and merge vertices, the other one wont merge


i would really appreciate some help. Please! :) thanks