Yes, it is possible to start learning both Chinese and Japanese simultaneously to build a basic understanding of each language.
During the initial months (1-6 months), you can use various apps, YouTube videos, and engage in conversations with native speakers, tutors, and other language learners to develop your language skills.
However, after this period, you may want to focus on one language for full immersion training. Immersion is essential for mastering a language, and splitting your attention between two languages might slow your progress. Determine the best places to learn Japanese and the best places to learn Chinese, then choose which language to prioritize.
Keep in mind that you may need 12 months or more to become fluent enough in one language before restarting your studies in the other. By focusing on one language at a time, you’ll be better equipped to achieve a high level of proficiency in both Chinese and Japanese.
Roughly speaking you can achieve A1/A2 learning both languages simultaneously for 2 to 6 months and that will be a basis to start a full immersion course that will then bring you to B2 or even potentially C1 within 12 to 18 months in one language and then you can do the same for the other language. In other words, learning both Chinese and Japanese will require a major lifetime investment on your side if you are a native speaker of English or other European languages.