I learned them concurrently, translating lua in Programming in Lua into the equivalent moonscript. It's a pretty strait-forward translation and is definitely doable. I already had experience with a reasonable variety of languages before learning it though, and for people still getting into programming, I think Lua is a simpler and still awesome language.

I don't think it's possible to learn moonscript without learning lua concurrently, since lua makes up so much of the underlying semantics. So I think the choice is either (a) learn lua, or (b) learn lua and moonscript for some nice syntactic sugar.