Taking hot backup of slave node in postgres (master-slave config with repmgr)

I have a #postgresql setup with master-slave configuration using repmgr. im
struggling with taking a consistent snapshot backup (pg_start_backup) of
the slave node, this is due the fact that the slave is in recovery, hence
read-only mode.

i need both the master and the slave snapshots to speedup the recovery
process (if taken only on 1 node, in case of recovery the slave needs to be
rebuild from scratch). any advice of how to take a consistent snapshot of
slave node? (master is no problem).

currently these are the approaches im checking:

1.shutting down slave node and take a cold snapshot backup.

1.

pg_start_backup on master and hopefully it will reflect consistent state
on slave (with the WAL shipping) - any confirmation on this one?
2.

stop replication, take hot backup of slave using regular methods and
then starting replication again - as far as i tested, not possible because
as the replication is stopped, each node becomes independent and has its
own timeline, thus require the nodes to resync in order to function again
in master-slave config.
3.

make a switch over before taking the backup - dont like this option,
dont think is a correct approach to make a switch-over each time i want to
take a backup.

anything i missed? have you implemented it using different methods?

thanks.