I would like to use these two mods together, but it's not precisely clear if I need to use Neveda Skies AND Neveda Skies - URWLNVified -or- if I should just use the latter.

Could someone tell me the best way to sort these two mods to work optimally with each other?

You need the "NSkies 0-6 Data" package from http://newvegasnexus.com/downloads/file.php?id=35998 and the "Nevada Skies - URWLified" package from http://newvegasnexus.com/downloads/file.php?id=39686. No other esp file should be used then the one in "Nevada Skies - URWLified".

Honestly there's a better mod out than URWL. It's the http://newvegasnexus.com/downloads/file.php?id=39619. This mod is used both inside and out, I use it for the inside (It even has URWL for an option but you don't have to have the mod loaded). I have Nevada Skies for the outside.

If you must use both Nevada Skies and URWL together, make sure you have the patch and it must be last on the load order. There's another mod I use for the outside, http://newvegasnexus.com/downloads/file.php?id=38196. It does have one floating tree by the Mojave outpost but aside from that the textures are awesome.