Any tank and any healer is a strong 2box combination. You'll get various responses saying why a pally or a warrior might be better than an SK, or why a shaman might be better than a cleric, but the overall differences aren't large enough to play something different than SK/Cleric if that's what you want to play. In general, paladins are seen as the strongest of the tank classes with their ability to hold aggro on groups of mobs the easiest, while also having a useful buff and great self heals and a group heal over time. Shadowknights still tank just fine though, and you won't really be tanking more than 2-3 things at a time until you are a high level with good gear. Clerics and shamans are each considered more powerful/in demand than druids, but any of the three are fine for 2boxing.
You'll be perfectly capable to duo all the way to 65 and beyond, but of course getting some DPS/Utility classes to group with you will allow you to kill faster and more at a time. You'll also have the ability to split up large groups of mobs while you are 2boxing by yourself once you get feign death on the SK. This is awesome for sniping a named that you come across, or breaking up a tough room so you can camp it easily.
Tanks can be rather expensive to gear up if you try to get the best of the best of buyable items, but you can still make enough money while leveling to afford good gear. The really expensive items don't really start until level 55 generally, and at that point you'll be able to grind in areas that provide good money in addition to experience.
Races really don't matter much, especially if you aren't a hardcore min/maxxer. Dark elves look the coolest, Iksar probably have the best racial for a tank, and the cleric races are all pretty well balanced compared to each other. (At least as far as I am aware; racials were changed recently but I seem to recall that they are all pretty even now).
The primary stat points for the classes are going to be wisdom for the cleric and pretty much all of the melee stats along with intelligence for the SK. I still wouldn't suggest putting your starting points into wisdom as a cleric though, since it's pretty easy to cap once you get to a higher level, while stamina or other stats are going to be much harder.
I'd really advise you to look at Adepts while you level. They are mini-raid mobs that all have a certain maximum level that you can engage them at (Ranging from level 1 up to the 50s). They are a great source of gear and unique items, and if you're leveling untwinked getting adept loot will make your life a lot easier.
