I'm a far cry from an expert as I just started dualboxing not more than a month and a half ago, but I would suggest (as stated prior) the Warrior/Druid and the Priest/Warlock. Reasons I give for this are the Priest/Warlock combo will add much DPS and healing when needed. It will supply you with a tank (void) and if your Priest goes shadow, it ups your Warlocks overall DPS as well. The Lock can place soulstones on the Priest and create Healthstones while your Priest can up your Stam on the Void and both toons.

As for the Warrior/Druid combo, your Druid can supply your Warrior with GotW to up his armor and stats as well as act as either an Offtank or DPS, not to mention his HoTs. Your Warrior, on the other hand, will successfully pull off the aggro from your Druid if such a case arises.

That's my thoughts, hope they help or atleast recap what others have told you or you already knew, haha.

Happy multiboxing!