Married to both Andreja and Sarah at the same time, the steps you have to do to achieve this is:
1. Flirt, romance and marry Andreja first, and max out her affinity.
2. then flirt and romance Sarah, once you start the romance Andreja becomes angry.
3. Speak to Andreja and if your persuasion is high enough you can convince her to forgive you.
4. go to Paradiso and marry Sarah, and don't invite members of Constellation to the wedding.

Both Andreja and Sarah will give you marriage dialog, and neither will get angry with you over this if you follow these steps.