In order to execute Cisco ASA commands on Standby unit, use failover exec as prefix.
failover exec is used to execute commands on a specific unit in a failover pair, the syntax is
failover exec {active | standby | mate} cmd
active – means the command is executed on active ASA unit
standby – means the command is executed on standby ASA unit
mate – means the command is executed on failover peer
asa/pri/act# failover exec standby sh run webvpn
webvpn
enable outside
anyconnect image disk0:/anyconnect-win-4.2.05015-k9.pkg 1
anyconnect enable
tunnel-group-list enable
error-recovery disable
Remember that any configuration command that you execute on the standby unit are not replicated to active unit or active configuration.