You can set the default value of the second property on the Page save/create/publish the event. Like below example
Here is a commonly referenced blog that demonstrates dependent drop-down lists.
Its important to note that newer versions of Episerver have changed things a bit, so please make sure you read the newer Comments in that Blog post, and refer to this Forum post.
Hope this helps.
In my page type, I have two fields Theme and Branding
I want to set value of Branding based on Theme on Content Creation.
I am trying to do this in SetDefaultValues method but it's not working