we are mainly interested in the cardiomyocytes from neonatal rat hearts. For this we isolate the hearts, digest them and preplate them in T75 flasks for 75 min in the incubator. We then transfer the supernatant to another flask. Many of the remaining and attaching cells are fibroblasts. You could detach those cells to get the fibroblasts. They won't be 100 % pure though.
Attached you can find a protocol. We used a certain kit, but this can be exchanged with any other isolation kit. I was refeering to day 2, point 10. We keep the cardiomyocytes and discard the fibroblasts in the flask. You do it the other way.