Depending on the weight of the knit I ...
A) serge first then double needle. I find that gives the edge more stability and makes my stitches consistantly nicer.
B) on finer/lighter knits I've been known to serge 1/2" lastin to the edge prior to turning and hemming.

I like the look of a wider double needle but if I'm dealing with lighter weight knits I find that a double needle that is enough smaller than the width of my serging so it fits within the serging is a much safer bet for me.

Hope you find something that works for you.

ETA: on the really light weight knits I've been known to opt for the last years trend of unfinished seams just to keep my sanity.