So you decided to level a char together with a friend? Or maybe you haven't decided on it yet, but just had the thought. Or maybe you don't have any friends to level with, oh well that's tough. In any case, here are some pointers as to what to think about when leveling with another person, because no matter how much you love/like eachother, problems will arise (if they don't, you're freaks). I've leveled alot of chars with Love, druid-druid, hunter-hunter, lock(me)-priest(him), lock(him)-rogue(me) and now most recently a rogue(me)-boomkin(him). None of these have made it to 80 together, for various reasons.
Level of Dedication
First of all, and probably most importantly, decide before you start out how dedicated you are about this business. If one intends to level the char as their next 80 or even main, and the other just feels like it could be a fun hobby on the side, you're already in trouble. The first problem that arises is namely that of different engagement levels. One wants you to go level that char, so you can both keep on doing it together, because he had some spare time and just accidentally dinged 10 levels ahead of you. As soon as your levels start spreading too far away from eachother, risk is big that you'll end the project all together. The less dedicated will definitely not feel like its worth the time to just grind levels alone to catch up, and the more dedicated will feel like you're spending to little time with your char whatever you do. Well I hope I have made my point clear. Make sure you both intend to spend the same amount of time and dedication on your chars, otherwise the whole "doing-it-together"-thing will soon be but a memory.
The second step should be to choose a nice class-combination. Not because some are better than others, but because different combinations will actually determine how easy it will be to get a group, succeed with difficult quests and etc and therefore change your playing experience. First of all, you should both play a class you enjoy to play of course. Don't force yourself to play something boring just because it's a nice combo like "I know, you tank and I heal and we'll get groups all the time!" "Yeah but I wanna dps...". If you intend to get these chars to 80 as new alts, maybe a good combination is less important. You'll have to go with whatever you want as your new alt.
Overall you can have three different setups;
Active-Active aka Dps-Dps
Active-Passive aka Dps-Healer/Tank
Passive-Passive aka Healer-Tank
Any of these setups work ok and are good and bad at different things. To this comes that different classes are faster and slower. Combining a slow and a fast dps hasn't proven to turn out good in my experience, but it all depends on if one of you is content on doing most of the work and the other is content with mostly being boosted (at least for the first 70 levels). Casters are generally slow dpsers until level 50ish, and melee are fast. Tanks are also generally fast, and healers slow (at dpsing that is). Casters can outperform melee at about level 60, but generally melee stay the "better" dpser until 70+ (the exception is rogue that suck until level 75 because of the lack of aoe). Combining two non-healers or non-tanks also means you have to be better at cc and stuff to survive. At low levels most classes can do some tanking though, or have pets who can. The only classes that can't tank or heal at all are rogues and mages (they have to use their magic cooldowns to survive).
There will also be a big decision to whether you two (or more!) are going to play the same class or not (or rather the same gear class or not). I do recommend that you play different gear classes, unless you're both fully equipped in BoA- gear by which the rest of ones gear matters less. Otherwise there will be grumpy faces when your friend gets that really nice gear piece that you would've killed to get your hands on (and still can since your friend probably is close to you). If you're in different gear classes you can both be genuinely happy for eachother when gear drops, which fosters good spirits and less throwing of things.
Because some classes are easier to level than others, I'd say that the best combination is paladin-paladin or druid-druid and the least good probably is mage-mage (because of the constant downtime). As paladin-paladin you'll both need plate, but one can need tank/healer plate and the other dps plate. Or with druids one can use caster-gear and the other one melee-gear.
Level of Equalness
Equalness includes several factors. But generally I'd described as; make sure you both have the same opportunity to get money and gear. If you're having your friend over from another server just for this, and you happen to have your main on this server, you have a greater opportunity to equip yourself with boa-gear and pimp yourself with nice boe-gear (and get boosts from friends etc). Make sure whatever you have comes your friends way as well. And the other way around of course.
This is especially true when it comes to gear. If the one has full boa-gear and the other doesn't, not only will you gain an uneven amount of exp, one will suck and the other won't. Feeling like the sucky, limp leg in a partnership definitely isn't fun. And I've already mentioned the necessity to have somewhat equal classes. If one for example plays a mage and the other a paladin/hunter, one will do all the killing and the other will do all the drinking (and water conjuring).
Have you ever had any good/bad co-leveling experiences?