If i have an outcome in mind, that happens.
If I can discern that my players don't much care, and I don't have a stake in the outcome, and it's an evenly-matched battle, I'll just roll a high/low die or have 1 opposed roll decide it.
If I can tell my players are invested in 1 or both NPCs, then what I've done in the past is have the players roll for the NPCs. It changes from an offscreen battle to a "cutscene" of sorts, and the players stay engaged.