There isn't a way for users to programme Keynote to use a specific slide master for any given project.
Thanks for your effort to answer the question Gary. This is all I needed to know.
The rest of your reply is of course completely subjective and is not helping me answer my question.
Custom themes must be selected manually, after all thats what using a custom theme with custom slide masters is all about.
The whole point of creating a custom theme is to create a unique design for a particular presentation. I certainly don't want Keynote to always use the second slide master when I add a second slide, that would defeat the purpose of a customised design. I decide what slide master to use, not Keynote.
I would say that the whole point of creating a custom theme is not to create a unique design for a particular presentation. If you wanted to do that, you'd be lot more efficient to design the actual presentation. The whole point of a custom theme is to create a custom design that you want to use more often. The purpose of themed templates is to save you time by letting you set default designs and functionality. And as someone who creates dozens of presentations a week for a number of clients I use templates to customise the look of these presentations to each client. But that's just my opinion.
While we're sharing opinions. I certainly do want Keynote to always use the second slide master when I add the second slide. The functionality of the Apple created themes to automatically switch to a different master page after the first title slide is worthwhile to me because it saves me a bit of time. In my experience it is also a completely logical function. After all, in none of my presentations I use two title slides in a row.
You and I obviously have different use cases for keynote. That's why it is great we can customise our templates. And to go back to my original question: I saw a functionality that benefits my use case, being used in Apple's default templates. The fact that it is possible in their templates led me to belief I should be able to define this behaviour in my templates as well.
And after all of that, regardless of your opinion on how to use custom templates, we have established a simple fact:
it is not possible.
Again thanks for your time to help me resolve this question