In community property states such as California, two people do become one when they marry. Under community property law, both spouses equally own everything they earn or acquire while they're married. Anything owned before the marriage or acquired after spouses legally separate, as well as assets received by inheritance or gift, is separate property, exempt from community property law. This is a big factor in dividing marital property in a divorce, but it affects estate planning as well, especially when you have children from a former marriage.
When a spouse dies intestate in California -- without a will -- his surviving spouse inherits all the couple's community or marital property. The surviving spouse already owned half this property, and she is entitled to the other half as well. The decedent's separate, exempt property is subject to division with his children, both those from a former marriage and any children the spouses had together. If the decedent had only one child from a prior marriage, and if the spouses had no children together, his spouse and his child would each receive half of his separate non-marital property. If the decedent had more than one child, his surviving spouse would inherit one-third of his separate property, and his children would share the remaining two-thirds. Depending on the length of the marriage, it's possible that the majority of the decedent's estate could be community property. If he left many children, they might each receive very little.
Omitted spouses and children are those not mentioned in a decedent's will. However, they may still be entitled to inherit from him. His surviving spouse still owns her half of the couple's community property. However, the decedent's half of their community property would pass according to the terms of his will. His omitted spouse is also entitled to the same portion of his separate property that she would have received if he had died intestate. If the decedent didn't mention his children in his will, including children from a former marriage, they also receive the same share of the decedent's separate property that they would have if he had died intestate. However, these rules only apply if the decedent did not deliberately leave them out of the will, either to disinherit them or because he provided for them in another way.
If spouses acquire assets in another state, then move to California, California's community property laws take over. These assets are quasi-community property, and they're distributed the same as community property if the decedent dies without a will or neglects to mention his spouse or his children in his will.
In the event of divorce, a decedent's children from a former marriage would not have any stake in community property he acquired with his second spouse. Community property law divides marital property 50/50 in a divorce.