Welcome ! Your post was very nice to read, thanks for that. Now to you questions :

Question 1 :

First, some general comments. I would avoid playing two mêlée classes. It is just harder to manage, not impossible, but harder. I would play two different classes. Two is still easy to manage (buttons and macro wise), and it's more fun. On top of that, one can look for synergies. One of the characters should be able to tank, at least to some extend (A Hunter's pet can be a rather good tank when paired with a healer). One of the characters should be able to heal.

Then it is a question of personal taste. Here are some things I would like to try:

- Warrior + Shadow Priest : The warrior can tank and do damage, the priest can do damage and heal. Very efficient levelling for sure.
- Rogue + Druid : Both can stealth, and that's great. The Rogue just does damage, and the druid can do all a druid can do (mêlée damage being hard considering how involving the Rogue gameplay is).
- Warlock + Shadow Priest : Synergy between the Shadow Priest and the Warlock (how is it called again ? Shadow Weaving ?). Warlock pets can tank, and the Priest can heal.
- Paladin + Shaman : Because of the Shaman's self-rez, and because then you will have a solid group base when you decide to upgrade to 5-boxing.

Question 2 :

What you describe can be achieved by a clever use of /castsequence. You should find a lot of examples in the Macros & Addons section. Then what to do is very dependent on class selection, of course. In general, in multi-class groups, I prefer to leave the more involving character as a main. Typically, I don't see how a Rogue could be played as an alt. Usually, the caster/healer has by far less skills that need to be used during combat. That becomes even more true with more different classes : the more classes, the most simplistic the approach of each class. At least that is my approach. Of course, some multi-boxers use a lot more of the potential of each of their classes, in particular through the use of clever macros, but I'm just not that good.

With only two characters, I would therefore not bother with too complicated macros or castsequences. I think your key+modifier approach is good. The thing is that while levelling, mobs drop so fast that complicated sequences are not often usable as the mob will be dead before step two or three.
I have been playing Hunter + Shadow Priest quite a lot. I was mainly playing the hunter close to its full potential, and the priest had about 3 dedicated buttons with sequences : DOT (Shadow Word - Pain, Vampiric Embrace, etc ...), DPS (Mind Blast, Mind Flay), Heal/bubble the Pet/Hunter.


Question 3 :

I never used a levelling guide. If you're playing under RAF, I just would not bother. Just go out and quest, avoiding collecting quests when possible. It goes so fast all the way to 60. Interestingly, I have the impression it goes faster and faster as you approach level 60 (I had that feeling with my three groups), so don't be turned down by the first 20 or 30 levels.

Post 60 is another story. I did it only with two characters at the time, and then the rule was simple : do every quest in the zone, then move to the next zone. I plan to do much more instances with my 5-men groups though. That will be in 3 weeks, when my RAF period ends. Until then, I keep on levelling "options for the future".