With two toons, you probably have a main which can be anything you enjoy playing, along with a good support alt.
For the support toons, ranged or caster is easier then a melee, but melee can work if you realize they are a bit of extra work.

If you're sure you will be two-boxing (two pc's or two accounts on one pc) and not going to go further then that, set up is easy.
Let's say your main is some kind of a tank, your alt is some kind of a dps/healer hybrid.
In this case, the alt needs a few macro's where they assist the main and deal damage, preferably break their dps moves down and match up with the mains moves that have the same cast time or cooldowns.
Also have a few heals set which heal the alt specifically and which heal the main specifically.
Then you match up abilities with shared cooldowns and set it up so a keystroke (keybind) does something on both screens.

If the alt will be doing some dps but will need to heal too, you might use /castsequence reset=combat !Dot. The ! key says to only cast it once, no matter how many times the macro is pushed.
That way, your main can push their dps button a ton, and your alt will support with a Moonfire (as an example), but then be free to heal without casting dps spells when they want to be healing.






I'd recommend setting up with either a focus or party leader system. The Focus free, Leader free, Target free system is better, but is a lot of work too.
Focus or Party Leader systems are a lot easier to set up, especially if you're only two-boxing and might not go further.
These systems allow either of your toons to be the leader, and with software such as keyclone (others too) you can set it up with picture in picture, so either toon can be the main.
Your PIP switch keys, will also be warcraft switch macro's.

If using focus, the key to bring toon A to the front (PIP) will also make them the focus on both characters. Similarly bringing B to the front PIP will make B both toons focuses.
Both toons will use "/cast [target=focustarget] Ability Name", so either can lead. If you added another two boxes down the road, the system does not need adjustments aside from more leader switch macro's.

If you using party leader, its slightly more complex in that your switch key promotes the new leader to leader. All toons have a promote key for each toon.
The promote key for other accounts (ie, not themself) will "/script PromoteToLeader("Name of Toon")" and on line two, "/changeactionbar 1"
Their own macro will read, "/changeactionbar 2".
So with two toons, you have two switch keys. Each has the actionbar2 for their own switch key and each has actionbar1 and promote for the other toons hotkey.
If you add a third toon, add another switch key which would be a version of the Promote/Actionbar 1, so each toon then has 1 of their own and 2 for switching leaders.
Then you put abilities for each toon on bar 2 (Shift 2 in wow) without any form of assist.
Put the same stuff in bar 1, ie Moonfire on key 3 in both bars, but on bar 1 use assists.... /cast [target=party1target] Moonfire" or line 1 "/assist [target=party1]" and line 2 "/cast Moonfire".
When you lead, you're on bar 2 and don't assist since you're the leader.
When you follow, you assist the leader. The first version will not target what you're casting at, but you still cast at the leader's target. The second version targets the leaders target and then casts at your (leader's too) target.
This is a little more complex, but fairly easy still. The advantage is focus is free for your use.

Both systems are in the wiki or forum searches in support forum will get a lot more details.