Where I went to medical school, the EM physicians had triple coverage from noon to 11pm, double coverage from 6am-noon and 11pm-2am, and single coverage between 2am-6am. IM and FP residents rotated through the ED (no dedicated residency), but no IM, FP, or surgery attendings staffed the ED as attendings. All were EM-residency trained and ABEM board certified. Annual ED volume was about 65-70,000.
The peds ED had a pediatric EM attending single coverage. Peds residents rotated through the ED. Pediatricians and peds EM attendings staffed the ED, occasionally adult ED attendings with no peds fellowship would staff if they were short. Not sure of the peds volume.