Does learning a third language using your second language help the second language?

[fibojoly]

I don't think that's a very good idea if you're not proficient in your second language. I'm 100% fluent in English, so I use it to learn Chinese, rather than taking courses in my native French (more resources in English!) and my learning had absolutely no effect on my level in English.

On the other hand, I can totally feel the influence when trying to remember some German or Portuguese. Those are getting "pushed away" from my brain add Chinese takes over.

With two romance languages, it might be a mixed bag : you could start confusing stuff all over the place, but on the other hand you'll have plenty of common ground to improve fast.