My sourdough starter split in 2 on the 3rd day. What should I do?
Sourdough during the first few days can show pretty erratic behavior. I wouldn’t worry, just stir and feed as planned. Continuing the culture with just the foamy top would probably also work. 

Reasons to start over would be visible mold (fluffy or colorful spots) or if there’s no bubbling. Neither is the case here.