How do I create a table of contents for both orientations of a publication ?

This should work fine provided you to the following:

1. Create separate paragraph styles for the portrait and landscape versions

2. Generate a separate table of contents for the portrait and the landscape versions

3. You need to enable the option "include book contents" to include the data from all files in the book

Bonus tip: you can define your styles in 1 of the book documents and synchronize them to the different other documents.