I started learning Russian while trying to learn basic Japanese for a while. The fact that they are very different languages helps avoiding confusion while learning some vocabulary and greetings, however if you arrive at the basic grammar with their rules...it becomes confusing fast, even if you are someone with good memory and attention to detail. I'm not saying it can't be done but it's difficult even for me who speak many languages. I would recommend advancing faster with the more difficult language and learning only basics with the easier language until you have some practise. Good luck!