The only and the most significant common thing in Chinese and Japanese is the usage of Chinese characters(Kanji). But they use simplified ones in China; traditional ones in Taiwan; middle way in Japan. The pronunciations of Kanji are highly related, but not the same at all. The vocabs in Kanji, some are shared, some are slightly different, and some are totally different. Other things, such as grammar, are totally different. Yet we Japanese understand Chinese if written to SOME extent because we share Kanji. But I don't know if it will benefit you or not. When you get advanced in both languages, you may find that there are some shared cultural things, such as Buddhism, Confucianism, Chinese classic literature, food etc., behind the language. But it would not be a matter until much later, I guess.

Hello Andrew, The phonetic structures of both Japanese and Chinese have a very slight similarity, that makes them way different. This similarity is due to the Japanese borrowing some of the older Chinese characters when creating their phonetic system. They borrowed them , yet modified many in structure and phonetic sound. So you could learn them at the same time nevertheless, but bear in mind that their similarity is marginal and will not very much facilitate the process of learning. On the other hand the similarity between Portuguese and Spanish is significant. Many words are very similar, to the extent that learning one language will help you recognize the words in the other easily due to their approximate spelling of many words.

when you write , some Jpanese words are similer to Chinese, but when you speak , it total different