The mage/lock combo is great. Locks bring alot of utility to the team that mages don't, and that's pretty much self-evident so let's not derail the thread too much on the question of "why".

One of my teams is always played with two friends. One provides the tank (pally), one provides the healing (priest) and I provide the DPS (2 mages, one lock).

It's a great team.

Cast sequences are pretty much down to personal choice but I have mine as follows. Bear in mind that they're only level 41 here, so I don't have as many spells to worry about as you might:

Opening sequence:
Mage: Pyroblast (long cast, no sequence).
Lock: CoA, Corr, Immolate.

I just hit the opening move button three times. It's no big deal.

Nuke:
Mage: Scorch, Scorch, Scorch, Fireball, Fireball, Fireball
Lock : Shadowbolt, and... erm.... oh yeah, shadowbolt

I use the same sequence on both mages just to keep them in sync.

My lock is destruction specced, having been distinctly underwhelmed with afflication damage output at low levels, and I hit Conflagarate manually because you often end up with a stalled cast sequence otherwise (resisted, bad timing, whatever).

"I need a shard" end-of-fight nuke:
Mage : Arcane Missiles
Lock : CoR, Drain Soul.

Instant nuke:
Mage: Fire Blast
Warlock: Shadowburn.

I used to have a FB->CoC->Blast Wave insta-nuke sequence for the mages, but found it was wasted more often than not due to range / positioning.

Works for me.