HA configuration

I can also see a Serial failover cable is plugged in between these two LTM using HA ports but in configuration I can only see network failover is setup.

there is no configuration for serial failover. anyway, you can use tmsh command to check serial failover cable status.

root@(ve11a)(cfg-sync In Sync)(Active)(/Common)(tmos) show sys failover ?
Options:
  cable  Displays the status that the failover daemon detects on the serial cable from its failover peer. It also shows what the failover peer detects on the serial cable. An active BIG-IP will see a zero from its failover peer. A standby BIG-IP will see a one from its failover peer.
  |      Route command output to a filter

To test this configuration we can disable 1.4 ports of both LTM and can manually failover to verify this setup. Do I need to disable this serial cable as well?

you have to remove serial failover cable when testing network failover.

If you configure a BIG-IP high-availability pair to use network failover and the hardwired failover cable also connects the two units, hardwired failover always has precedence; if network failover traffic is compromised, the two units do not fail over because the hardwired failover cable still connects them.

sol2397: Comparison of hardwired failover and network failover features