Manuel Caldera, 24, and Jose Ceja, 20, were arrested Saturday after the two victims identified them as the men who had sexually assaulted them.
According to French, the alleged incident began at about 11 p.m. April 10. The women told investigators that the men had picked them up for a consensual encounter, but then allegedly got a gun, threatened the women and forced them to perform sexual acts.
"They feared for their lives," French said.
The women reported the alleged crimes Saturday morning and police arrested Caldera and Ceja later that day, French said.
Caldera has been arrested in connection with three prior rapes, including a prostitute rape case from 2003 in which a cold-hit DNA match identified him as a suspect, French said.
Two of the victims in the prior cases were prostitutes and the third was a family friend, French said.
French said investigators were investigating Caldera as a possible serial rapist.
According to court records, he has been charged with six felonies, including two counts of forcible rape, in connection with the alleged April 10 assault and an alleged assault from May 2008.
Ceja has been charged with one felony count of forcible rape, according to court records.
Both men remained in county jail Friday. Bail has been set for each at $625,000.
They were arraigned earlier this week, but did not enter pleas.
They are scheduled to return to Contra Costa County Superior Court in Richmond Thursday to be assigned attorneys and enter pleas, according to court records.