Is it possible to have both a epigastric hernia and diastasis recti?

Yes, it is possible to have an abdominal wall hernia such as an epigastric hernia along with diastasis recti.  A hernia is a defect in the abdominal wall that allows intra-abdominal fat or bowel to protrude, and diastasis recti is a separation of the rectus abdominis (a.k.a. “Six-pack” muscles), both of which can present as a bulge.
In order to best serve you and flatten your abdomen, both should be addressed in some fashion at the time of your abdominoplasty.  
Good luck!