Few newbie questions

[hovercrafter]

To merge the verts, select all them and then do a merg verts. You can also select all verts on the model and merge them then go to the channel box and click on the merge node and decrease the merge threshold to 0.01 and that gets the verts that are right on top of each other (or too close for a good mesh).

[Schalldämpfer]

If my vertex overlaps another object, and I select it, it automatically selects the other object as well as the vertex, too...
And to merge the centerline vertices is there a way to do them so that I can maintain the symmetry? I want to work on only one side and have the other side mirror it, but it seems like to merge the center vertices, you need to combine the two objects.

[hovercrafter]

If your getting the mirrored object selected while in vertex mode on the original object, then you are most likely doing an add to selection, dragging and not actually selecting any verts. If there aren't any selectable components of the current mode in the mesh and you do an add to selection, then it will think you are wanting to add the other mesh to the selection.

The simple work around is to put your instanced copy onto a layer all it's own. Once you do that you can make the layer a reference and not worry about selecting any part of that copy.

As far as merging the meshes and doing symmetry work, you can't. Once you merge the two meshes, there is no longer and instanced side. Just leave them as separate meshes until your are done with the modeling.

The other method is to create a Sub-D proxy. In the options set your mirror to full and set the direction. Set the diviisons levels (exponential) to 0. Then select the new subd mesh, put it on a layer and hide it. There is a merge threshold in the SubD proxy you can adjust that will basically keep the center verts merged to each side until you move them out of the merge threshold. That's the closest thing to the Symmetry modifier in 3Ds Max.