I'm currently married. I had an affair with another man. I'm not about 10 weeks pregnant and expecting twins. Based on the current due date/conception date, I'm 99.9% positive that the father is the other man, not my husband.
I'm going to assume that, at the time of conception, you were living with your husband. If that's not correct, please say so because it's an important facts.
I understand that there's a 6 month waiting period in our state,
What that means is that your divorce cannot become final until at least six months after the date of filing.
Am I correct in understanding that since my husband and I will still legally be married at the time of the birth he will automatically be considered the legal father?
No. First, if you're currently 10 weeks pregnant and your husband filed for divorce last week, there's a possibility that the divorce will be final before the date of birth. Second, while some states make the date of birth the only relevant relevant, this is one thing that California sort of gets right. In general, "the child of spouses who cohabited at the time of conception and birth is conclusively presumed to be a child of the marriage."
Fam. Code section 7540(a). In other words, unless you were living together at the time of conception and are living together at the time of birth, the presumption of your husband's paternity won't apply.
Does his name have to be added to the birth certificate or can he refuse?
You will control what goes on the birth certificate, and I'd suggest you not identify a father until and unless paternity has been conclusively established.
So once they are born and a test is performed, what will happen if it shows that my husband is not the biological father?
Not really sure what you're asking here. Hundreds of things will happen (just as hundreds of things will happen if he is shown to be the father).
If he is proven to be the biological father, how do I get him listed as their legal father instead?
Listed where? On the birth certificate? You can contact someone at your county clerk's office and ask about this.
And because there are still several months to go and he could change his mind about willingness to be involved, what would happen if my husband is listed as the legal father by default, proven to not biologically be their father, and then the other man decides he wants to try to avoid the situation and refuses to accept paternity? Can I legally force him to take a paternity test?
You'll have to file an action to establish paternity and convince a court that it should order this guy to submit to a paternity test.
I'd strongly urge you to retain legal counsel to help guide you through this.
Apparently you think this is a joke.
No one thinks this is a joke. Some folks who respond on boards like this will focus on legally irrelevant things like your and your adulterous partner's cavalier attitude toward your marriage vows, and you responding by trying to defend your adultery will serve no useful purpose.