This isn't a post on how to gem or gear your disc and holy specs respectively, but what you should do if you want to collect one gear for both of them. Ideally I'd say, don't do it ;) It's way better to have one gear for each spec. But the world isn't always as ideal as we wish it was. So if you've ever thought about dual speccing double healing specs, like I have, there are some things you should know. Mixing Disc and Holy gears is probably more difficult than mixing elemental/resto or boomkin/resto specs (except for the fact that boomkins/elementals need hit). In fact, pre-patch it was easier to combine shadow with holy or disc than combining disc and holy. Generally one spec needed mana regen more than the other, and this is definitely the case between disc and holy as well.
Although alot has changed with the new patch, there are still alot of differences between what stats holy and disc want to prioritize that makes gemming and collecting a combined set for both specs difficult. The more you buff the one spec, the more you sacrifice the other spec.
For example, pre-patch stats like haste and spirit were less useful for disc, but really useful for holy. That meant that if you wanted to collect alot of haste and spirit for your holy spec, you'd have to sacrifice crit and intellect that were the good stats for disc, and vice versa of course. Now post-patch we have the same problem. My gear is very heavily gemmed towards disc, and I tend to avoid gear with spirit. That means I have low haste and low mana regen, but high spellpower and really high crit. It doesn't mean my holy healing will suck, but I will oom considerably faster than a "normal" holy healer. As mentioned the best thing would be to keep separate gears of course. And that is still my dream goal. It's just a little tricky to gather a second set of the same gear when there are plenty in my guild who'd like their first set. This issue might even become even bigger once Cataclysm comes, since we might find that holy really needs alot of spirit for mana regen. Right now mana is less of an issue for any healer honestly.
So if you have to choose one spec to gem and gear for, which one should it be?
If you find yourself using one spec considerably more than the other, say some 70%-30%, you should gem yourself towards that spec of course. This is one of the reasons I have chosen to gem towards disc instead of holy, since I mostly play disc nowadays (now we recently got another disc healer in our guild and I find myself playing holy more and more, maybe I have to regem soon...). But if you find yourself playing both specs about the same, you have to identify which of the two specs that would suffer less from having the "wrong" gems.
Like mentioned - playing holy with disc-gear I will be low on haste and mana regen. On the other hand I will have nice crit, which is really important for holy healing. I will also have alot of spellpower, which also is good.
If I am disc with holy-gear I will be high on haste and mana regen. This means I will probably be overcapped on haste (less so post-patch though) and have mana regen for no use since Rapture and Archangel keep me from ooming anyhow. Holy stats on a disc priests are therefore more wasted than disc-stats on a holy priest. Holy stats on a disc-priest are overkill. They don't bring anything extra to the character. Disc stats on a holy priest means you will be short on some important stats, but instead you will have access to other important stats. None of the disc-stats are wasted on a holy-spec.
Because I have mixed these two gears I have kept myself pretty high on haste. Higher than I probably would've been if I'd only played disc. Because even if the disc haste cap is fairly low (but considerably higher post-patch) haste is still not a wasted stat on a disc priest. Especially now there is plenty of room to lower the casttime on heals like Heal and Greater Heal, which we are supposed to use regularly.
So if you have to choose to gem and gear for one collected gear for both specs, I'd definitely go with the disc stats. They used to be intellect and spellpower, but since they're now both in one you're pretty safe off with only intellect. This means you will have mana issues when in holy, but hopefully this drawback will be less of a problem than being overcapped on haste and having unecessary regen when disc!