Basically what I'm interested in is where there any research whether the linguistic similarity of two languages that are learnt simultaneously contributes to the effectiveness of learning. For instance, let's say following hypothesis: when you learn two closely related languages  at one time it's less efficient because you mix up similar vocabulary and confuse syntax and grammar. Let's say we state another hypothesis: learning related languages in parallel speed up learning. 


Just to protect this question of being closed as dup: [this][1] question asks about the studying of similar languages - the author even claims that they "not interested in the general study of multiple languages, but specifically those which are similar" -  and the only passage related to my question is that one blogger strongly advises agains learning related languages in parallel - but this claim is not backed up by any scientific research. 

[This][2] question is about learning similar languages but not necessarily in parallel. It's also does not about comparison of learning two similar languages in parallel compared to learning typologically very different languages in parallel. 


  [1]: https://languagelearning.stackexchange.com/questions/1/are-there-any-studies-which-address-the-effectiveness-of-studying-multiple-relat
  [2]: https://languagelearning.stackexchange.com/questions/2548/are-there-any-studies-on-the-effectiveness-of-learning-typologically-similar-lan