it's been said but I want to make sure it's clear
Warrior/Cleric
Wizard/Cleric
is the best duo possible. however you will need to add a bard in there eventually. if the wizard dies you'll need the warrior to rez the wiz if you're going to duo which is why the warrior gets cleric sub.
Now the reason you'll need a bard eventually is because of the speed and your wizard will run out of mp a lot at 60+ even with the largest pots.
The most basic euro party is something like this
Warrior + Wizard + Cleric
a good euro party is something like this
2 Warriors + 3 Wizards + Cleric + 2 Bards
so going duo is fine, i do it with my friend I'm a warrior/cleric my friend is a wizard/cleric both level 80 and we own, but make lots of friends, it is indeed faster if you have a nice big party of people that know what they are doing. However, big parties of people that don't know what they are doing = crap.
full 8/8 parties can go to mobs 11 levels higher than them and fight, with a duo like you want to do, I'd suggest you go to whatever mob your warrior friend can tank effectively, in a duo normally they will be mobs your level.
When you hit level 40 you'll be able to put up recovery division for your friend then switch to wizard and nuke away, even after 40 your friend might have trouble. Your tank friend at levels past about 20 should be tanking about 3+ party mobs at the same time since your nukes should be AoE, hitting 3+ mobs per nuke is better than 1 at a time.
err i can keep typing a bunch of stuff for a year, if you have any more questions ask away, i can elaborate on anything you need.
Part 2 Delvling
ok don't sp farm at such a low level, it's better to really play the game maybe at a 2~3 gap and get a feel for it before getting into sp farming.
what delvling is what it sounds like, when you die you lose from .5% -> 2% depending on your level, at low levels you lose 2%. now you just die 50 times and you lose a level.
The reason people do this would be if they found a level they want to farm at because they get sp really fast at that level, however if they happen to level up from let's say 24->25 and they realize i'm getting 1 sp less per kill but I'm killing at the same rate... wtf... then they go ahead and lose a level and continue farming.
For your Tank Friend
for European characters their skill mastery determines the level gear they can wear.. unfortunately if your tank friend goes at a 9 gap he's going to find it hard tanking mobs even if they are his level because his gear will be 9 levels below that of the monster, in a good 8/8 full party it won't matter but in a 2/8 party it will unless he has exceptional gear like me xD (bragging)
err, i wanted to say something else but i forgot

i might edit it later.