Yes, that's possible. Be sure to grab the prefetchpathelement when you add the first subnode, so you're adding the subpath node to the same parent node.
So do NOT do this:
// bad, doesn't work:
IPrefetchPath path = new PrefetchPath((int)EntityType.CustomerEntity);
path.Add(CustomerEntity.PrefetchPathOrders).SubPath.Add(OrderEntity.PrefetchPathOrderDetails);
path.Add(CustomerEntity.PrefetchPathOrders).SubPath.Add(OrderEntity.PrefetchPathProducts);
but do:
// good:
IPrefetchPath path = new PrefetchPath((int)EntityType.CustomerEntity);
IPrefetchPathElement ordersNode = path.Add(CustomerEntity.PrefetchPathOrders);
ordersNode.SubPath.Add(OrderEntity.PrefetchPathOrderDetails);
ordersNode.SubPath.Add(OrderEntity.PrefetchPathProducts);