Support for SermonSpeaker 5.x is in bugfix mode only, the same as it is for Joomla 3.10.
I will not add any new features to this releases.
Please upgrade to SermonSpeaker 6.x and Joomla 4.x, which is better anyway.
Is there currently a way to tag multiple speakers? Say for a guest speaker as well as the normal one?
Would this be difficult to implement if it is not currently supported? It looks like speaker is stored as int(10) in the database so maybe it would be possible to use a different field (custom?) and maybe call it Guest Speaker or something like that?
You can only assign one speaker per sermon. However there are currently two "custom" fields in the database which can be used for such cases. They can be enabled in the SermonSpeaker Options in the "Detailpage" tab.
You will need to create language overrides and possibly also template layout overrides to adjust the name and possition of it.
If you can wait for Joomla 3.7, it will have a big feature "Custom Fields" which will allow to create own fields. SermonSpeaker will support that feature as soon as J3.7.0 is released.