I know there are many pieces of software including webapps with the aim of teach either Chinese or Japanese.
There are also app makers which have versions of the same app targeting Chinese learners and Japanese learners separately.
But as somebody who is learning both languages, and having recently met quite a few other people while travelling around Asia who are learning both, I'd be very interested in finding a piece of educational software that aims to help people who are learning both.
There are two main areas where knowledge in Chinese and Japanese learning have a very large overlap that could be leveraged by the right software to help bridge the gap between different teaching traditions:
Characters (Hanzi, Kanji)
Many characters are identical between the languages
Many characters are related variants
Common shared vocabulary due to extensive borrowing from Chinese into Japanese
By learning the usual sound correspondences between the languages it can be easier to remember and even to guess the pronunciation and meaning of new multi-character words.
The main benefit would be the reduction of time and effort in vocabulary learning.
- Only that the app be designed to help people learning both languages.
I'm assuming that this would be a small niche with not many apps, hence I don't feel adding more requirements would be beneficial.