Comparing two Corpora using Topic Model
In my eyes, this is not a valid approach.

Note, that there is not one unique topic model (given some parameters like the number of topics and the algorithm for topic modelling) for a corpus. Different runs with different random seeds will give you different topic models for the same corpus.

So, any comparison comes down to a comparison of specific topic models, but not to a comparison of the corpora.

An approach with some better validity is to combine both corpora into one super-corpus, create a topic model of it, and than investigate the distribution of the topics with respect to the sub-corpora formed by the original corpora 1 and 2.