What I would hope is that the LAS will be the answer to your question (and I think this is VR's intention). If you make the decision with any of the 3 tanks to have more offensive skills on your bar, you will do decent DPS. If instead you decide to mix in defense or prioritize defense, your DPS will suffer accordingly. With the classes being incomplete and things like the Mastery system being turned off, it is really impossible to answer your question right now anyway. But the good news there is that if VR nails this, you will be able to play the tank of your choice and decide from session to session how much DPS to dial in. One of you can be the DPS one day and the other can soak the damage, and then you can switch.
DL has been generally considered the highest DPS of the tanks, though that's not guaranteed to hold all the way to Release.
As far as a duo, the general wisdom is to have 2 different classes that compliment each other. Like a tank and a healer. But playing what you like is also widely advised hehe.
Grab a shaman to level with. Debuffs and dots, plus Heal over time + Buffs = win
AND Windstrider movement buff!