When you start inputting the different surnames, you'll realize the current structure can handle even this. What a scoundrel.
Leave the Bigamist's last name at birth (LNAB) as is. (That will be his 1st wife's "current name", unless she remarried.) Hopefully you have a marriage record to support this.
Put the Bigamist's second surname as an "Other Last Name", unless he legally changed his name at some point. If he legally changed his name put that as "Current Last Name."
For his second wife, treat her as you would any other second wife..... but her "Current" surname would be the Bigamist's second name. Hopefully you have a marriage certificate for this marriage!!!! And birth records for the children, with the Bigamist's name.
In the biography, you'll have to explain.
Sorry for your discovery.