I'm not sure why theres a gap coming up - but u should not try to "merge" an original and a dublicate. The dublicate is only for reference while modelling. So if you want to work this way with smooth proxy - you should only apply the smooth proxy to on of the sides (the original).

However, I highly recommend you to use "Connect Poly Shape" instead, which work like smooth proxy, but with much more features. Version 3.5 can be downloaded here: https://www.lightstorm3d.com/tools/fr...PolyShape.html

I hope this make sense.

Kühl

Btw, if you have problems with CPS - let me know.