If you're planning to duo with this a fair bit then perhaps a /therm or /dark would go well with traps. Poison is more ore less just more potent single target versions of the same debuffs. They're invaluable for AVs, but general missioning you may find something with some AoE heals might be in order.
Demons do pack their own pocket healer, the ember demon. Although his aura heal does have a hefty recharge on it and ember demons are notorious for having very bad endurance problems.
A /dark will give you a nice AoE heal (two in fact once you get Dark Servant). Thermal also gives plenty of heals and resistance as well as some extra debuffs.
Don't go poison unless its your GF toon and your playing the Traps. 
Personally I would make toons that stand on there own so if you bail from the game or she bails from the game either one of you is not stuck with something that might be sort of lacking because you relied on the other so much.
Whomever takes the Traps is going to be able to obtain positional defense cap. So solo running on setting of 8 or in a 8 person team they will do fine. But the poison player will not be able to do the things the Traps player can. It can be a bit disheartening.
I would suggest Dark because Darkest Knight can give you defense cap as well. Granted I am not a big fan of having a toggle debuff needed for increasing my defenses, since things can go wrong, like detoggle due to stun or endurance drop, Darkest Knight Anchor running off, etc. But Dark is still powerful and between Triage Beacon increased regen and Darks heal its nothing to complain about.
Pain isn't bad as well, you can get some limited form of Defense cap mingled in with regen that pain offers. So you could play a tankermind role as well. I haven't finish building up my DS Pain and slotting to really tell you how it is working for me so I can only speculate looking at the numbers and such. Again not a big fan of talking about builds I don't personally play because I can't give you an honest feel for the build.
Even double traps is great. You will both have kick butt toons. You could provoke a AV between each other like a ping pong. You get a big hit from an AV that sneaks in. You stop attacking and she provokes the boss onto her to give you chance to regen from the double Triage Beacon going. You can alternate dropping a Triage Beacon so you will always have one running.
The more I think about it I think double traps would be the way to go. If your that into it you could actually get positional defense cap at around lvl 26 or 27. It would cost you thought to slot 6 Gaussian into Tactics for each toon and 4 slotting defense IOs into FFG and Maneuvers.
Here is the build as an idea. End result at level 26 you both would have 50% to positional defense as would anyone else on your team. Which includes your pets. I would put the lockdown proc in poison traps and watch tons of mobs being held between the poison mist or the proc. Put the -res proc in Acid Mortar. 3 Heal/Recharge IOs in Triage. The next 24 levels will be very easy for the both of you when your together.
Now it is at this point you can either go the route of being self sufficient and obtaining personal defense cap so when solo your good. Or if your that sure this will be a duo effort then you can sacrifice that and pick up a few different powers that you normally would not. Its a tough call to make.
Villain Plan by Mids' Villain Designer 1.90
http://www.cohplanner.com/
Click this DataLink to open the build!
Level 48 Magic Mastermind
Primary Power Set: Demon Summoning
Secondary Power Set: Traps
Power Pool: Leadership
Power Pool: Speed
Power Pool: Leaping
Villain Profile:
Level 1: Summon Demonlings
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- (A) Gaussian's Synchronized Fire-Control - To Hit Buff
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- (15) Gaussian's Synchronized Fire-Control - To Hit Buff/Recharge/Endurance
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- (21) Gaussian's Synchronized Fire-Control - To Hit Buff/Endurance
- (21) Gaussian's Synchronized Fire-Control - Chance for Build Up
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Level 1: Brawl
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Level 4: Ninja Run
Level 2: Swift
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Code:
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1. Why Soft Cap is Important : http://dechskaison.blogspot.com/2011...important.html
2. Limits: http://paragonwiki.com/wiki/Limits
3. Attack Mechanics: http://wiki.cohtitan.com/wiki/Attack_Mechanics
4. Rule of Five: http://wiki.cohtitan.com/wiki/Rule_o...e_Law_of_Fives
My girlfriend and I made this combo tonight and was debating if it would be good or not.
I can see there are a ton of debuffs but lacking with heals...any theories?
In time, I'll come to forgive you.