We consider an M/M/1 status update system consisting of two independent sources and one server. We derive the average age of information (AoI) of each source using the stochastic hybrid systems (SHS) technique under the following packet management with self-preemptive serving policy. The system can contain at most two packets with different source indexes at the same time, i.e., one packet under service and one packet in the queue. When the system is empty, any arriving packet immediately enters the server. When the server is busy at an arrival of a packet, the possible packet of the same source in the system (either waiting in the queue or being served) is replaced by the fresh packet. Numerical results illustrate the effectiveness of the proposed packet management with self-preemptive serving policy compared to several baseline policies.