Additional PHP Version - server field empty (3.1)

This could well be user error (RTFM), but when I add additional PHP versions on an ISPConfig 3.1 cluster (Ubuntu 16.04). I have a choice to which server the additional PHP version should apply. If I select the master server, the server name appears in the PHP versions list. If I leave the default, which turns out to be the slave server. The list shows the PHP version without a server name.

1. Should I be adding PHP versions twice, once for each cluster member. If so, why isn't the slave server PHP version shown by server name?

2. Should the additional PHP version only be added one (to either master or slave server), as it's assumed both servers have the same PHP version installed?

Thank you for a great piece of software, much obliged.