I believe you can do that with a subsearch. Maybe something like this?
sourcetype="mtab_executer" OR source="unix:useraccounts"
| rex max_match=0 "(^|\n)(?<usb_key>\/\S+\s\/\S+)"
| eval user_id=coalesce(user_id, uid)
| eventstats latest(user) AS user BY user_id
| convert timeformat="%d/%m/%Y %H:%M:%S" ctime(_time) AS c_time
| stats latest(c_time) AS Mount, latest(user) AS user, latest(user_id) AS user_id, count BY usb_key
[ search sourcetype="linux_audit"
| transaction startswith="type=SYSCALL" endswith="type=PATH"
| convert timeformat="%d/%m/%Y %H:%M:%S" ctime(_time) AS c_time
| stats latest(c_time) AS Umount, count BY name
| fields Umount ]
| table Mount Umount user user_id usb_key
... View more