Are you on medication? There's no problem with St. John's Wort and evening primrose, but there might be a problem with medication and St. John's Wort. Evening primrose should be taken with meals that contain some fat, as it is mostly fats. And yes, you can also take Omega 3 oils. Evening primrose primarily contains omega 6 oils. As for dosing herbs, which would include St. John's Wort, they should be taken two to three times a day depending on the dosage to keep them in your system. That's not necessary with the fats, since they store in the fatty tissues.
Yes it is okay to take both. I would take SJW in the morning and primrose oil in the evening. Also consider Omega-3 and vitamin B complex supplements for optimal results.
M4