The service for cousins dual Irish-US citizen Ashley Donohoe, 22, and Olivia Burke, 21, from Dublin, was taking place in Sonoma.
The bodies of four other Irish students will be repatriated on an Aer Lingus flight to Dublin on Saturday night.
They are due to arrive at Dublin Airport on Sunday morning.
On Friday night the relatives of LorcÃ¡n Miller, Eimear Walsh, Eoghan Culligan, and Niccolai (Nick) Schuster received their bodies for the first time at a joint service in St Columba's Church in Oakland, California.
Their bodies lay in repose in open caskets until around 22:00 local time.
After Saturday's joint funeral service for Ashley Donohoe and Olivia Burke, Ms Burke's remains will be flown home to Ireland on Sunday evening.
The balcony collapsed during a 21st birthday party in the early hours of Tuesday in the city of Berkeley.
Five of the six people who died were Irish students based in the US as part of a work exchange programme.
The victims were:
Meanwhile, the conditions of two of the people who were injured in the Berkeley balcony collapse have improved.
Conor Flynn and Jack Halpin are being cared for at the John Muir Medical Centre in Walnut Creek.
Hospital pastor Fr Michael Gillen described the pair as inspirational and said they should make a full recovery.
Investigations continue into why the fourth-floor balcony collapsed.
Engineers say water damage may have caused the structure to give way.
Authorities said 13 people were on the balcony when it collapsed.
A book of condolence is to be opened at Belfast City Hall on Monday.

The request was filed in Lille and a ruling is expected in 48 hours.
French President Francois Hollande wants to close the Jungle and move the estimated 9,000 migrants it holds to reception centres across France.
The Jungle has become a powerful symbol of Europe's failure to cope with African and Middle Eastern refugees.
The migrants there live amid squalid conditions, many of them hoping to enter the UK illegally by hiding on lorries crossing the English Channel.
President Hollande had said the camp would be dismantled by the end of the year but some aid workers said this week that the demolition could start as early as Monday.
The charities said they were not against the demolition but were concerned that authorities were rushing the process without adequate preparation.
"Contrary to what we were told, the plan is to dismantle the Jungle very quickly and do it in a matter of days," one of the groups applying for the injunction, Secours Catholique, said in a statement.
"It will be impossible to ensure serious preparations that cater properly to people's interests."
But Pascal Brice, the director of Ofpra, the agency in charge of handling asylum requests in France, insisted the closure would go ahead.
"Calais is over, it's finished," he said. "What is at stake is for this message to get across and for people to realise there's no point in going there, that it's a dead-end."
The government has already started moving refugees to new locations in other parts of the country ahead of the demolition.
Figures for the total number of migrants in the camp are disputed. Recent figures released by the French prefecture state that there are between 5,684 and 6,486 people. Aid agencies however, estimate that there are about 9,000 people.
Fresh concerns have also been raised about the hundreds of unaccompanied children in the camp.
French charity, Terre d'Asile, says there are now 1,290 unaccompanied minors at the camp, an increase of more than 400 in two months.
The unaccompanied minors are aged between eight and 17.
However, other aid groups, including Medecins Sans Frontieres, have questioned the accuracy of the survey, stating it was unscientific.
A joint funeral mass is being held in California for two of the six students killed in an apartment balcony collapse in Berkeley on Monday night.
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Aid groups have asked a French court to delay the closure of the Calais migrant camp, the "Jungle", arguing authorities are not ready to relocate refugees.