Hmmm, if you are working in a later version of max use the symmetry modifier no hassle this automatically welds the verts touching together. Make sure you check the weld verts box. Also if your second side isn't visible click the flip check box.
Or...mirror the first object on x. Somewhere in the rollout editable mesh or poly there is an attach button click this then the second object now you can weld your verts. *Note: with this method your your objects can't have faces where they are being welded.
*Instance: anything you do to main object happens to all its instances