If you are a 3dsmax user (version 9+) you can check out TexTools which is a bunch of scripts in 1 toolbar. Anyway in TexTools there is a symmetry tool
that mirrors either one side to the other one (select half of the faces to mirror) or just select the edge mirror and average booth sides.
It works like this:
Just make sure one half looks good (relax some parts, tweak some by hand and rectify some edges so you don't get distorted pixels later) and once done simply select that half using face selection and simply mirror it to the other side.
Another tool in TexTools (added in 3.1) is a snap to pixels button which will snap your selection (edge, verts or faces) in the UVunwrap editor to pixel boundaries. Its used like this:
The button for this is located on the Selection Transform Bar that pops up if you open the UVunwrap editor from the TexTools Toolbar. This tool is useful for very low texture work where snapping to pixel size really matters.
Also I would suggest working in a square UV size and once done with your 2:1 half just use the UVW xForm to extend it or simply use the recently added UV canvas tool in TexTools 3.2
Which just uses the xForm UVW modifier but does the math for you so you don't need to dig with reversed axis, 0-1 ranges and alike things.