I can see a variety of ways to pull off something like this. I think the core concept is a very cool one, but also something very hard to design well.
Here are a few examples (ie, randomly made up powers) of ways I see this sort of thing working:
Generalized Combo Attacks
These would be powers that don't key off a specific combo power being used, but simply gain a bonus off of a condition a wide variety of powers could inflict.
Ignition | Wizard Attack 17
With a word, your opponent bursts into scorching flame that sears them to the bone, then fades away.
Encounter * Arcane, Fire, Implement
Standard Action | Ranged 10
Target: One creature
Attack: Intelligence vs Fortitude
Special: If the target is suffering ongoing fire damage, you gain a +2 bonus to the attack roll.
Hit: 3d8 + Intelligence fire damage, and the target is dazed until the end of your next turn. If the target is suffering ongoing fire damage, they immediately take additional damage equal to that ongoing fire damage.
So, with a power like this, you gain a bonus effect if the opponent is suffering from the right condition, thus providing synergy between several powers. The problem with this is that it is awfully hard to balance - if you make it so the power, when it gets the bonus, is better than other powers of its level (to compensate for the bonus being conditional), then it might end up too powerful if the condition is easy to arrange. If you make it so the power is too weak without the bonus, then it could end up a useless choice.
Finding the right power level between the two is certainly possible, but a lot harder than balancing most powers.
If you do successfully balance this, however, then you can design a variety of combos to hurt enemies, leading with attacks that synergize together and gain bonuses when used together. It makes for some nice tactics, but again, is a tricky thing to arrange.
Combo Attacks
So, setting up powers that complement each other works fine, but making two powers completely a combo is more difficult.
The best way I see to do so is synchronized attacks, which I think is what Azigen is looking for (as this would most resemble the combo attacks seen in Final Fantasy and other such games.)
I puzzled for a bit on how to set this sort of thing up, but then realized the system already has a great way to do so: Immediate actions.
Here's a look at how that might work.
Steiner's Sword Magic | Knight of Alexandria Attack 11
Your strike your opponent with your blade, setting up a perfect conduit for your companion's magic to strike them as well.
Encounter * Martial, Weapon
Standard Action | Melee Weapon
Target: One creature
Attack: Intelligence vs AC
Hit: 2[W] + Strength damage, and the enemy is dazed until the end of your next turn.
Special: If an effect is causing this attack to deal fire, cold or thunder damage, increase the damage by 1[W]. If the effect causing this is Vivi's Magic Support, increase the damage by 2[W] instead.
Vivi's Magic Support | Small Black Mage Attack 11
As your ally strikes your foe, you enshroud their weapon with magic, blasting the enemy at the same time.
Encounter * Arcane, Implement; Cold, Fire or Thunder
Immediate Interrupt | Ranged 20
Trigger: An ally makes a melee attack
Target: One creature attacked by your ally
Attack: Intelligence vs Reflex
Special: If the attack that triggered this was Steiner's Sword Magic, gain a +2 bonus to the attack roll.
Hit: 2d6 + Intelligence fire, cold or thunder damage, and the enemy grants combat advantage to the triggering attack.
Effect: The attack which triggered this deals fire, cold or thunder damage (of the same type dealt by this attack) instead of its normal damage type.
Now, these may or may not be balanced, but it strikes the kind of back and forth action that seems to be what a combo should be all about. Here is how the attack sequence goes:
1) Steiner steps up to the enemy and declares he is using Steiner's Sword Magic.
2) Before Steiner attacks, Vivi interrupts with Vivi's Magic Support.
3) Vivi's Magic Support gains a +2 bonus to hit since it was trigger by Steiner's Sword Magic.
4) If Vivi hits, the attack deals some elemental damage, and makes the enemy grant Combat Advantage for Steiner's attack.
5) Whether Vivi hit or not, Steiner's attack will now be dealing elemental damage.
6) Steiner now takes his attack, with a +2 bonus from the Combat Advantage.
7) If Steiner hits, he deals an extra 2[W] damage.
Now, you could obviously take this a step further - the type of damage might change what condition he lands, there might be an extra bonus if both attacks hit, etc. But this seems simple enough to allow for some cool synergy without requiring too much to keep track of.
It also leaves both abilities still useful even when not specifically used together - Steiner's attack can still do decent damage on its own, inflict a useful condition, and gain a bonus if other elemental effects are used (such as a flaming weapon or something similar.) Vivi's attack can complement any melee attack, making it elemental damage and giving a bonus if Vivi hits.
So... combo attacks are certainly doable, and very cool conceptually. I especially favor having the combo based around one triggering attack, and other parts of the combo as immediate actions to 'join in' on the combo.
But, as can also be seen, combo powers will inevitably be both complex and somewhat challenging to balance, so should be done with care. But I can definitely see some cool possibilities from them, and some rather innovative things they could be used for.
