Well, as to your question about the best duo pair.... i guess it is all opinion, so I will give you my opinion on it. I took some time and looked at all of the classes. I wanted to find the best of each class in the game.
From what I saw, I decided that the templar was the best pure healer in the game. I put templar and inquisitor about the same, although I give templar a little bit of an edge because of some of the spells they get.
Now, when I was looking at tanks, there are many good tanks also. But, for pure tanking reasons I believe that guardians are the best. Yes berserkers can do more damage and go berserk, and yes paladins can ward or heal themselves to make more aggro. But, I think that guardians are the pure "brick wall" of the tanking class.
For this reason I two-box a templar and a guardian, and I have found it pretty effective.
If you two choose any healer and any tank, it should work out. You can tailor the characters to your playing style, which is what it is all about. You will have an advantage because you already have the group almost set. Just find one more DPS to trio with ... or find one more healer and three DPS and you can take on anything.
Good Luck,
Good Hunting,
Healfirst - Nektulos