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.
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.