For the collection professions you can have skinning on an alt and have it skin something your main is targeting. I don't think this works for mining or herbalism as you can't actually target the node that you gather from (you click on it instead). Skinning macro is fairly simple:

/cast [target=focustarget] Skinning
(change targetting to whatever you will be using for your toons, so you could /assist your main or [target=party1target] and you can also use the [dead] condition, though I don't think it matters, as you can't skin a live target).

For other gathering skills it's easier to have them on your main. I have herbalism and mining on an alt, mining shows up on my mains mini-map (as she's also a miner) and the nodes are plenty big enough to click on the little window that my alts have. Herbalism is much harder for me as I find the nodes very tricky to target on the little screen (and my other toons tend to stand on top of it).

I'd suggest an addon (I think gatherer can do this) which shows you positions for possible node spawns. I'm not using this as I don't think my current machine could handle this, but if you are getting a tracked gathering skill on an alt then you'll benefit from having a way to track possible nodes from your main (my herbalist is lagging behind on skill as most of the time I forget to look, or can't see nodes for her).