In Standalone LB, like to implement Active / Standby facility between 2 servers

You can use Priority Group Activation on your pool to achieve this

https://support.f5.com/csp/article/K7065

Use pool member priority groups to configure a pool with an alternate server or group of servers using a lower priority value than the primary server's priority values.

Set the Priority Group Activation setting to Less than..., and then type the number of members below which the system activates the group.When the number of available members in the highest priority group drops below the minimum number, the BIG-IP system distributes traffic to the next higher priority group.

Note: When the BIG-IP system activates a lower priority group and its pool members are processing connections, active and persisted connections continue to use the lower priority group pool members after a higher priority group becomes available. Connections to the lower priority group pool members continue until the connection has completed or the configured persistence timeout is reached.