In an ideal world where you have free choice of what to carry [I know, I know, in an ideal world it wouldn't be necessary, stop nit-picking] and to carry is practical, you would either want to carry a second weapon that complements the first, or one that supplements the first.
A comlimentary weapon provides options and abilities not readily applicable with the primary, but which increase effectiveness of the primary.
A bali is light and fast, mostly effiective in close. So a compliment should add weight or range.
Ideal compliments would then include batons, flails, and throwing weapons. Batons include things like tonfa (modern versions are often referred to as nightsticks), collapsible batons, or kubotan (roughly the size of a pen, used on pressure points or just to add mass to your fist.). A flail could be as simple as a manriki chain or a yo-yo. Or just pick your favorite throwing weapon, from shiriken to bootknives to billiard balls.
Suplimentary weapons basically continue the abilities of the primary. Basically, this means carrying paired weapons. In this case, two identical balis.
But a word of advise: To carry a weapon is to invite trouble. Whether you carry it openly or tucked safely in a pocket.
If you carry it openly, you are saying, "I'm BAD! Try me punk!" This is why many martial arts prohibit wearing their uniforms outside of class.
If you carry a hidden weapon, you will tend to think of an encounter in terms of that weapon. "If your only tool is a hammer, you begin to see every problem as a nail." Not only can this cause trouble to escalate to an unhealthy level where things get completely out of control, but it also limits you in such an encounter. People just have a basic tendency when holding a weapon to forget that they (and consequently, their opponents) have numerous natural weapons (hands, feet, knees, etc.) which opens a combatant to dangerous defense gaps.
Far better to learn to defend yourself with your wits and your hands, and have the blade as a backup if all else fails.
Now, I'm not saying I never carry my bali. But I never carry it as a weapon. I carry it as a knife, a tool. Please, remember this.
Sorry to go preachy on you. I'm bad about that, ask Mykl. But it is something that should be considered. Balisongs (and knives in general) have a bad reputation mostly because people don't treat them responsibly. There is nothing inherently wrong to a weapon. However, the spirit in which a weapon is held can pervert it to evil purposes--not to sound melodramatic.
End of sermon.
