Check luggage weight against airline limits.
Excess baggage is charged per kilogram and it compounds per segment, because bags are re-weighed at every check-in on a multi-leg itinerary — three kilograms over on a two-segment trip is charged twice. That per-kilogram structure means the fee escalates fast, and beyond a few kilograms it frequently exceeds the flat fee for an additional bag. Comparing the two routes explicitly is worth doing at the kerb, because a second bag also gives you a fresh full allowance rather than just permission to be overweight. The one thing to check is the per-bag weight cap, which stands regardless of how many bags you buy.
Excess fee
Fee = (bag kg - allowance kg) x fee per kg x segments
Comparison
Second bag cost = extra bag fee x segments; choose the lower
Because on many itineraries — particularly when separate tickets or different carriers are involved — bags are re-weighed and re-charged at each check-in. A single through-checked bag on one ticket is usually charged once, but a self-connection means paying the excess again.
Beyond roughly four or five kilograms over, often yes, and you get a whole additional allowance rather than just tolerance for the overweight one. Check the per-bag weight cap though — most carriers refuse any single bag over 32 kg for handler safety, whatever you have paid.