Nothin? 
I have a few lvl 50 stalkers myself, and I usually do pretty well teaming up with a brute/tank. With the AOE damage from spines, you would want someone to take most of the aggro while the stalker dishes out all the damage.
I'd go brute. As far as your powers, you want something with a lot of res/regen. You can't really go wrong with Super Strength / Willpower.
If you dont like that combo. You can go with Super strengh, Elec Melee, Claws, or even Street justice.
Secondarys, Willpower, Elec armor, Fire amor, Invuln, or you can match your buddy and go with Dark Armor. Regen is nice and new, but if your going to be taking aggro, just know that you need to be more proactive with your heals.
On the defence side Super Reflexes is my personal fav, but bear in mind that when you get hit, you get hit hard. Shield defence is good fun too, decent defence with some res back up.
I hope I helped somewhat, i'm sure SOMEONE will disagree.... but we all know how forum trolls work.
Just about anything with good AoE Debuffs/Strong Team Buffs or AoE Aggro Control.
So, AoE Support Corruptor/Defender or Team Support Corruptor/Defender... AKA most any of them.
Or a Tank/Brute/Scrapper (Scrapper with Taunt/Damage Aura). Again, most any of them.
Hell, even a strong Control Dominator would be good... or a Ranged Pets Mastermind...
I think that means most anything would be decent.
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If you guys are mostly duoing, then I would suggest just another Stalker. If you play a squishy, you are going to get a lot of aggro. Stalker is not good at holding aggro. Stalker can take hits but not holding aggro for teammates.
I used to duo Stalker with my twin brother. He is Claw/Will and I am Spines/Nin. He would start the battle with Shockwave and I follow up with Throw Spines. His Claw attacks provided us a lot of safety.
If you don't mind making a Stalker, give him a try. :P
With two Stalkers, you guys can pick your fight and if there is one boss, you two can handle him/her very quickly.
What's left is to normalize all Assassin Strikes and improve Stalker's old sets (Claw, MA and EM)! You don't need to bring back the missing PbAoE attack. You just need to make the existing ones better! For example, make Slice a WIDER and LONGER cone.
Hello,
I like to play CoX with my brother, and he is really into his spines/DA stalker. I was wondering what is a good duo for that build?
He is only level 10, and it would be for PvE leveling. We just want to wreck groups of mobs and have fun thrashing crap around lol.
So what AT and power set would you recommend to best compliment my brothers stalker? Im open to anything thats gonne pwn.