I would like to know the same thing. There are several languages that I want to learn through Rocket Languages: Spanish, French, Japanese and Sign Language.
I'm already familiar with Sign Language, but not really fluent in it so hopefully Rocket Sign Language would help a lot.
I was thinking of doing Rocket Spanish and French at the same time, but I felt that since the language is a little similar, I would get them mixed up. So I thought that I would invest a few months for each language.
I think it would be a smart idea to do one language at a time so that once you are comfortable and fluent in the first language, then you can move on to the next. I think it's better that way, but that's just me.
I would love to learn 2 or even 3 languages at a time, but I think that might be too much to input.
But if anyone else has no problems with this, please share your experience as I would love to hear it :)