I wanted to figure out how to do this too.
Ended up mirroring the object, then combining them and then making a selection down the middle of the two objects (thereby selecting all the vertices needed). Then I simply did edit polygons -> merge vertices (making sure the default settings for it were on).
Might not be ideal but it saves a lot of time going round every face/vertice and merging them (which I had ended up doing previously!) :headbang:
p.s, that method was from memory so it might not be totally exact