Single tcpdump command for client and server side

You can use "p" interface modifier to capture flow (ie. both external and internal side traffic).

"Capturing traffic with TMM information for a specific traffic flow

Beginning in BIG-IP 11.2.0, you can use the p interface modifier with the n modifier to capture traffic with TMM information for a specific flow, and its related peer flow. The p modifier allows you to capture a specific traffic flow through the BIG-IP system from end to end, even when the configuration uses a Secure Network Address Translation (SNAT) or OneConnect. For example, the following command searches for traffic to or from client 10.0.0.1 on interface 0.0:

tcpdump -ni 0.0:nnnp -s0 -c 100000 -w /var/tmp/capture.dmp host 10.0.0.1

Once tcpdump identifies a related flow, the flow is marked in TMM, and every subsequent packet in the flow (on both sides of the BIG-IP system) is written to the capture file."

For more information see: K13637: Capturing internal TMM information with tcpdump