How bigip decide it's status

The failover cable is just an electrical signal. If you reboot the BigIPs, the first one to fully come up should be the one that goes active first. The second unit should then go standby as once it comes up it will already be receiving a signal from its peer.

If you plug in the cable when they are both active, I would presume that the first one to detect the failover signal would be the one to go standby. You should never have units in standby/standby (as in in normal use this configuration is impossible).

There is no way to predict this. Under normal circumstances it should not matter who is active and who is standby. If it does matter in your environment, I would suggest network failover which allows this configuration.