The Museum of Knowledge team is often asked if learning two languages at the same time slows down learning. One of the greatest challenges a language learner may face when trying to learn more than one language at a time is confusing words from various languages or speaking in one language and then suddenly making mistakes, by using vocabulary from the second language. For those who have already mastered learning a language, it is far easier to then commence learning a second language, to learn new phrases and vocabulary that are distinct from the first language.
There are of course similarities between certain Latin languages. Sentence constructions for those who learn French and those who learn Spanish are in some cases, structured in a similar way, with nouns, verbs and adverbs placed in a similar order within a given sentence. In fact some language learners may find learning two languages at a time to be very useful as the more exposure to languages as a whole, the easier it is to absorb techniques such as verb conjugation, gender uses, noun placements and verb tenses.
In conclusion, the Museum of Knowledge team would advise that if the speed of learning a language is the most important part of learning for a student, language learners should start one language for at least six months before embarking on learning a second language.