NUS-HEC Double degree more beneficial as compare to single degree of NUS or HEC alone?

with respect to Salary and career growth?

Of course this will expand your network, but that's only valuable if you see your business future using those networks,

It's good if you want to study in both places, and like Duncan says, build your networks in both places.

It's twice the length of either program though. I would think that lost wages during that second year would dissuade a lot of people. I doubt that having two degrees would increase your earning potential very much (if at all) over just having one degree.

It's not twice the length: you do semesters 1 and 2 at HEC and then 3 and 4 at NUS. SO it's the same length as NUS.

You can accelerate NUS MBA and study during internship to complete the degree in 12 or 15 months.