MySql Switching Masters During Failover with log-slave-updates
The warning about --log-slave-updates will only apply if you had multiple intermediary 'Master/Slave' servers.

The warning is this (my emphasis): 


  Then it will write updates that it receives from Master to its own binary log. When Slave 2 changes from Master to Slave 1 as its master, it may receive updates from Slave 1 that it has already received from Master


But in your scenario, Slave 2 is not changing masters, it will still point to the same Master 2 server it always was at.

So now, in case of Master 1 failing, you will need to do two things:


Make sure your applications point to Master 2
Follow the instructions on promoting one of the three slaves to be new Master 2
make sure to enable --log-slave-updates on the new Master 2