Unlike skydiving which involves leaping from an aircraft, base jumpers take off with a parachute from a fixed point, usually a cliff or a bridge.
The two incidents took place some 900 kilometres (550 miles) apart — one in the Alps and the other in the Pyrenees.
On Wednesday morning, a 34-year-old Swiss man living in the Mont Saxonnex ski resort was killed after jumping from a spot popular with base-jumpers in Haute-Savoie.
Firefighters said the man's parachute failed to open. He struck the cliff before hitting the ground, and was killed instantly.
The second victim was a 46-year-old man from Paris who jumped from the north face of Pic du Midi d'Ossau in the Pyrenees around midday.
Rescuers said the cause of the accident was also likely a problem with the parachute opening.
The man fell 700 meters (750 yards) and was found dead on arrival, according to firefighters.
A base jump is defined as a leap off a fixed high point — the name being an acronym for Building, Antenna, Span (such as bridges) and Earth (natural settings).
The pursuit carries high risks.
A Norwegian study published in 2007 concluded that base jumping is between five and eight times more likely than skydiving to result in injury or death.
More than 35 people have died base jumping in the Lauterbrunnen valley, a popular area for practitioners of the high-risk sport in the Swiss canton of Bern.