If you are able to 2box well, then there is no point in not doing so unless you do not like it. A bard and druid combo would be pretty nice as it would max out your damage shield and provide healing and crowd control. If I ever make an alt or steal someone's character, I want it to be a druid (not like I don't box one for buffs anyways~)
Bards AND druids can also solo very well. If you get bored with 2boxing and want to just solo some stuff, either class is great at what they do.
Some people will say box a cleric, but quite frankly, clerics are only really good for raids or for 2boxing very high end content. Overall I think a druid would be the perfect complement to your bard.