It is going to be lot easier to get two professions on your lvl 80. And for future: yes, it's perfectly fine to level up professions while levelling.
If you want to get two gathering professions, go for it. There are addons( like this one
that make it easier to get mining/herbalism combo that is more profittable. What it does is to change tracking every one or so second. So I suggest going with that for money.
However, it is very viable to get crafting profession paired with gathering one. Instead of buying mats on AH you will be simply getting them on your own as you level the other profession. It is cheapest way (well, without putting in any hard cash) as many lower level mats are very overpriced (mithril and thorium in particular) on AH. You can have friendly enchanter DE all pieces that you make and sell those. Or you can sell greens for decent money if you're patient.
If you plan to make another alt sometime, then I strongly suggest getting professions as soon as possible. You are simply wasting time by powerlevelling them at cap instead of working on them as you go.