In the realm of fetal intervention and surgery, you'll find a great deal of overlap between fetal therapy trained pediatric surgeons and fetal intervention trained MFM providers.
In its current state, the fetal trained pediatric surgeons (such as those who've graduated from CHOP, UCSF, TCH) will get involved to the degree that they are comfortable with fetal interventions in conjunction with the fetal intervention MFM team. The specific roles depend on the culture at the institution and the procedure at hand. For example, you'll find at some institutions that the pediatric surgeon and the MFM will perform the hysterotomy, and the neurosurgeon will close the MMC defect (in cases of ONTD), whereas it some centers (especially Europe), the pediatric surgeon may do the entire procedure themselves, and again in other locales the MFM does the procedure with the neurosurgeon doing the fetal repair. Then there are cases such as EXIT to resection (MFM does the EXIT and the surgeon the resection), laser for TTTS (MFM/FI generally does the entire procedure), aortic valvuloplasty where the MFM inserts the trocar in the LV and the Interventional cardiologist threads the coronary catheter through the LVOT
In summary, my response to you is: it depends on the location, the team, and the procedure.
If you want to go the OB route, then you're looking at a 4 year residency, 3 year MFM fellowship, and then some form of fetal intervention training.
If you want to go the surgery route, then as mentioned above, you're looking at a pediatric surgery track with some research and fetal specific training.
Hope this is helpful!