BIG-IP : configure 2-node pool as simple failover

You can use priority groups for this.

Set priority group for each pool member. Node 1 to higher priority (eg. 4), node 2 to lower (eg. 2). This is in Local Traffic ›› Pools : Pool List ›› 'your_pool_name': Members tab: Click each member and set the Priority Group in Member Configuration part (Advanced settings)

Then in Local Traffic ›› Pools : Pool List ›› 'your_pool_name': Properties tab: General Properties part set Priority Group Activation to Less than 1 Available Member(s)

This will activate the lower priority group (=node2 member) when monitor detects less than 1 available member in the higher group (=node1 member as down).

Note that you must have health monitors defined for the pool or members, so that BIG-IP has member status info.