Pretty much depends who's playing them! But it can be real nice to have a banjo & mandolin _without_ a guitar getting in there.

The conventional wisdom is that you should avoid combinations of instruments that have about the same timbre and tonal range--particularly harmonica and fiddle. But just to confound the experts, Mike Seeger sometimes plays both of those at the same time, in unison, for an effect that's altogether spooky.

Whereas if they both play different parts in the same range, it can sound weird and clashy. You can ameliorate this effect though if one plays high and one plays low (and the other plays the raggle taggle gypsy-o).

Willie-O