Just play what you think is fun
[You must be logged in to view images. Log in or Register.] You will have a better time. As long as the combo isn't crazy broken (warrior / rogue would be slow I guess).
Even double ranger you could be an archery duo! And use awesome bows to super kite things!!! Anything that you can outrun you could kill (I solo killed a sand giant at level 18 with my ranger)
Like SK can be a pretty good tank (self healing a bit) and mix it with a....necromancer!!! Who uses empathy spells to heal!!
2 necromancers would be pretty strong too...they can double heal each others pets (whoevers is taking the hits).
Or be clerics! And tank / heal each other!! (I was soloing with my cleric till 54, getting great exp too).
Or, be double monks!! Super bind wound team!! Or an enchanter and paladin!