I'm no expert on Deep Dungeons, but I did clear HoH for the mount, so I have some experience.

I'd say WAR. I've been duoing WAR/SMN with my wife. Nascent Flash is really handy to top off both of our HP from random damage that is taken. There was a lot of value when we stepped on a landmine during a pull with luring trap (good strategy...). Instead of panicking - Nascent Flash gives her a damage shield, couple that with AOE damage on all of these mobs and then popping Shake if Off for another shield/regen - all on a floor with no auto-heal.

That's not to say WAR/SMN would be easy mode. From a tank perspective, I think WAR gives excellent survivability to the duo.

As you'll find out, on floors X1 to X9, what kills you isn't auto-attack damage. It is being hit be telegraphed (or untelegraphed) mechanics. There is some tank privilege, but it only extends so far. Bring a Phoenix Down. It will buy you that one free raise if the SMN is taken out.