As usual, the answer is "it depends." =) What is your ultimate goal for the tradeskill? Are you wanting to make coin for yourself or make items for you/your friends/your guild to use? If coin is your goal, then two gathering skills seem to be the way to go.
I'm currently slogging through inscription on my shaman alt and find it interesting yet fairly unfulfilling. I essentially give away my skillup glyphs on the AH because nobody wants them and/or there are a bunch of low priced ones on the AH already. You could sell the herbs that you are milling for way more than you can get for the glyphs (on my server, but it seems to be most servers from what I've read).
Granted, my inscription skill is only 215, but I can't even give away most of the glyphs I make because they don't favor my friends' specs. Or they have better ones than I can make already.
At the higher level of inscription there are some very nice treats. Some day I will get there. But for now I'm using up a whole bunch of herbs that I could be selling for good coin in order to skill up on glyphs that I have to throw away in the AH in order to make room in my 32 slot inscription bag.
By contrast, when I leveled up alchemy on another toon I could always pass out any skillup items that I wasn't going to use myself, and sell the remainder.
Whatever you decide, good luck with it!