I say "Go for it!" It can be done. There are people who do it.
I am studying French, Spanish and (sometimes) Portuguese and this is difficult because of their similarities. I mix them up! It is also probably easier because of their similarities.
Language study is hard but you know two "difficult" languages already. Therefore, I think that you can study French and Japanese.
My recommendation is to find French-Japanese people who speak both languages. They will be very useful to you. It is also good to search for Italki threads with French people learning Japanese and vice versa. This will help you a great deal. It will only work if you are at least intermediate in both but try it.
The man who chases 2 hares catches none.
There are very specific adjustments our brain goes through as we learn a new language. Trying to learn different languages at the same time will just burn you out without giving any significant results.
The best thing you can do is flip a coin and pick one.
If you study french 1 hour a day and Japanese 1 hour a day, it will take you so many years to get any decent results.
If you study french 2 hours a day for 1 year, you'll be set for life. You can then apply the same rigor to Japanese.
The fact that you are willing to learn 2 languages at the same time show that you have great work ethic: if you decide to focus on just one language, you will definitely be successful.
If you do decide to learn Japanese, you can use these videos to learn faster: https://www.youtube.com/playlist?list=PL-E-O4DnY4Zul84y7dTVYcvu3KvCs4GQW
Angus, no problem man good luck with learning both.