Later on you can get instruments that double as a weapon too. But even when you just use normal weapons, their songs (sung) can still be pretty potent.
A bard left on auto pilot will not be all that useful though. Yes their songs will be quite a nice buff for your 'main' character, but only similar to just getting buffed by a druid or shaman buffbot or whatever. In other words, having the bard just following you around will be quite wasteful of the bard and wont make you all that much stronger than just soloing. Although if you can manage to do some stuff on the bard, their dps can be pretty good, especially if you assign their attribute points in the right way. (str/dex/int) And with practice, even though 2 boxing seems hard at first, in time you'll be able to go back and forth and use the second character almost as much as your main guy. The main issue I would say about a bard though, is that their DPS is not as good (or as easy to do) as a monk or a sorc or druid or whatever. What really makes a bard worthwhile is special stuff like the ability to mez stuff and charm stuff. But this can be really hard to do when you are 2 boxing, because time spent focusing on charming one mob and then sending it on another mob, is time you are leaving your main character completely unattended, which can be nasty. Especially later on when you do a dungeon and you end up with several mobs beating on you - as soon as you start trying to mez or charm something, the time it takes you to do that is often enough time for your main character to die. In other words, a Bard is really one of those classes that should really be played by itself so you can focus on all their many useful tricks.
And even if you do manage to box it incredible well, or even if you pick two other classes, as I've said before, 2 boxing is not perfect. In the lower levels the mobs are so easy and squishy that having 2 characters is barely even an advantage because one character can kill them just as easily, only now you need to collect all the quest items twice and spend double the amount of time gearing up your guys etc.. You also might still struggle to do places like Khegor's End with just the 2 characters because you miss a lot of important abilities at that level (ie, no Lull!), and its just a hard dungeon anyway. In the mid levels, having a 2 box really comes in to its own and you will be able to do a lot of the group and small group content that you wouldn't have been able to do alone. For example I did the whole of the Dargun's Tomb questline as a 2 box, apart from the very final battle for the weapon. But then going to the higher levels, it becomes almost useless again because the mobs are so much harder, you'll just struggle to make any progress with just the two characters.
I 2 boxed four different combos of characters and had lots of fun, but it always comes grinding to a halt in the 40's, because you end up fighting such beefy mobs that they just take an eternity to kill with only 2 characters. So I am now 6 boxing and it's amazing but it's also very stressful and requires a huge amount of effort, and it also is not for everyone because your PC needs to be able to cope and you ideally need to macro some things to help out too.
So if you really like the idea of a Bard, personally I would suggest playing it alone so you can be really good at it and do all kinds of cool stuff. You should be able to group most of the time too because the population is not bad. They can provide killer buffs for any class in their group, they can dish out really nice dps, and they can do crowd control as well as even a Psi. If you are determined to 2 box just for the fun of it or because you want to do some of the harder content by yourself (you can do hunters league etc. as a 2 box), then I would suggest picking something other than a bard to make life easier on yourself. And if your PC is good enough to 6 box (it's really not as demanding as you might expect), then you should give it a try, it's totally amazing, like a real time Baldur's Gate, and the bard is worth having around in a 6 box.