OK - I'll bite first.
Fire/Rad times two.
Fire/Kin x2
Fire/Nrg x2
Though TBH for levelling any two will do quite well, I'd suggest you steer clear of the Tank/Healer method as in CoH its not especially fun, though a healing controller might do. A Scranker (based on a scrapper) and a healer based on a controller with Sonic/FF wouldn't be so bad though.
Find six more people and run a Super Team, if on Defiant I'd join for a fire/rad (or anything/rad except mind), and some other things. And I can bring two at once who are already able to work well in a duo.
edit: I love to try a Grav/Rad Super Team, which from my theory crafting should be able to hold AVs permanently from 32 upwards. (Based on pet that concentrates on holds over pure damage.)
Most duo's work well together. Blaster blaster is fun as you will have to work smartly to avoid getting faceplanted but you do wickedly damage. Your secondaries can be used to keep the enemies away from you.
A scrapper scrapper combo would work well as well of course. Good damage, good survivability. Plenty of fun working close together sharing a joke or two.
Have duo'd two scrappers to 50 with another scrapper and it is indeed great fun
"Well, they found my diary today.
They were appropriately appalled
at the discovery of the eight victims
They're now putting it all together.
Women wrapped in silk
with one leg missing
Eight legs, one body, silk,
spider, brilliant!"
Myself and my wife are running 2 scrappers (DB/WP and BS/WP) and it's great fun. 
Happy to be on Defiant.
Global name @mereman
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i mostly do scrapper/scrapper duo, but i was surpised how much fun a scrapper/tanker duo can be, especially on lower levels
My first duo with my wife was a Blaster/Controller based around Fire, Fire/Fire Blaster and Fire/Empathy Controller. worked really well in synergy and was a hell of a lot of fun to play - carnage wherever we went 
U can try /dark scraper and sonic/ defender. The scraper will have his rez caped most of the time, although as u probably know /dark is a bit more end heavy.
My girlfriend and I are currently duoing an Electric/Psi blaster and a Fire/Fire tank. That's quite fun!
Edit: I know you said no tanks but I didn't read that bit til after I posted 
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Kalis Elfling - Inv/Axe Tanker. [i]Kevin is watching... Always[/i]
Proud member of [url="http://www.honourableunited.org.uk/"]Honourable United[/url]
@Tellis - I reserve the right to be ridiculous![/center]
I would suggest a controller/blaster duo such as fire/rad plus psi/psi. Should give you a nice combination of control and damage and lots of fun to boot.
If it has eyes, you can blind it, if it has blood, you can make it bleed, if it has a mouth, you can make it scream.
Basically a SG mate and me made a villain conceptual duo and pacted them we made a fire/shields brute and a Fire dark corr
so any advice or suggestions would be great thanks 
now we decided to do the same on heros but i'm having trouble coming up with a good duo that would compliment each other well, as for the concept anything that would fit science or tech would be great, oh and no tanks