Some years ago I was learning German and at the same time trying to improve my English. It was interesting, although demanding. I think translating, for example, one phrase or text from the languages you learn into the native language was a good practice. One of my students used cards to memorize words: she wrote a word in Russian, French, German and English. And absolutely agreed with Lora: strong motivation is very important.

strong motivation is the only way to learn even two languages at the same time. Just don't learn them at the same day. Today is for Russian. Tomorrow is for Chinese.

May be you should learn and consider the same sections of the languages at the same time and compare them. Find out for yourself differences and the similarities.

There shouldn't be a problem unless the two languages are very similar, then you might confuse them for one another.