The first boat, which capsized early on Thursday, had nearly 100 people on board. The second, which sank later, was carrying about 400 passengers.
Rescuers have pulled 82 bodies from the water and saved 198 people, according to an official with the Red Crescent.
About 100 people are still missing, said the official, Ibrahim al-Attoushi.
The boats sank on Thursday after leaving Zuwara, a major launch pad on the Libyan coast for migrants hoping to reach Italy.
The UN says about 2,400 migrants have died trying to cross the Mediterranean to Europe so far this year.
More than 100,000 others have landed in Italy, whilst another 160,000 have crossed to Greece.
The Libyan coast guard worked overnight on Thursday to search for survivors from the latest tragedy.
But Libya is poorly equipped to carry out rescue operations as the ships available to its coastguard are small, BBC North Africa correspondent Rana Jawad reports from Tunis.
At least 100 bodies were taken to a hospital in Zuwara, west of Tripoli, a resident told the BBC.
The victims included migrants from Syria, Bangladesh and several sub-Saharan African countries, the resident said, but the information could not be independently verified.
A detention facility for illegal migrants in Sabratha, west of Tripoli, received 147 people, an official told Reuters.
On Wednesday, the bodies of at least 51 people were found in the hold of a stricken ship off Libya’s coast.
They were picked up by a Swedish coastguard ship that also rescued more than 400 survivors – among at least 3,000 migrants saved that day.
The Swedish ship, Poseidon, docked in the port of Palermo, Sicily, on Thursday.
On Saturday, about 4,400 migrants were rescued from boats off the coast of Libya, in one of the biggest single-day operations mounted to date.
Many of those who attempt the journey are fleeing conflict or persecution, and set off from Libya in unseaworthy boats organised by smugglers. Libya has had two competing governments for the past year and is largely ruled by rival militias.