SK Bard is a pretty solid one, honestly. SK Shammy was the old school dual boxing king and would still be a decent choice even with mercs being an option.
In my eyes, either of the Knight tanks + utility dps covers the most ground. The thing about bard is while it's less dps than other combos, it brings all the utility and also covers CC basically as well as you'll ever need it, though if you've got a good camp spot, you can get by with just good pulls (which SK can do). Bard brings just a little of everything, which is hard to beat when you're only playing 2 characters.
Knight+beast offers an awful lot. Slows, spot heals, backup tank (albeit not nearly as good as the knight), decent personal dps, tons of mana regen. Bard I would say is number 1 for this, but this is a close second for me.
Mage+Enchanter can do a large majority of the group content in the game and has been able to ever since mercs came out.
Wildcard - SK+Wizard - With the massive inflation of named HP, this isn't as good as it was a few years back, but they had the burst dps to down named well before SK epic ran out back in TBM and before. All that stuff I said about utility goes right out the window with this combo, but it was a whole lot of fun when I played it by hand.
A ton of combos can work these days though, so in the end it's going to come down to - What do you enjoy playing the most? I know it's the tired 'heard it 100 times' answer but it's true. A buddy of mine has been boxing Warrior Enchanter for years. Before I joined RG, when I didn't feel like boxing my brother and I got through a lot of content with him on mage, me on ranger and no other outside characters.
My favorite combo for a duo has and always will be necro and shammy, unbelievable powerhouse group. Can run cleric merc and Wiz if you like, but i never did. Just the cleric merc. Let them dots eat! I eventually added a bard, which I know isn't best for that setup, but I love bard speed, plus increasing dot dps was on point! At the end of the day, play what you enjoy, and go from there.