The following diagram (figure 1) shows how to monitor traffic that is entering/leaving a VPC from two EC2 instances (A and B) in the public subnet and concurrently monitor intra-VPC traffic between two EC2 instances (C and D) in the private subnet. In this example, each of these instances (A, B, C, and D) are mirroring sources.

https://aws.amazon.com/blogs/networking-and-content-delivery/using-vpc-traffic-mirroring-to-monitor-and-secure-your-aws-infrastructure/