Basically what I'm interested in is whether there is any research on whether the linguistic similarity of two languages that are learnt simultaneously contributes to the effectiveness of learning them. For instance, let's say the following hypothesis is formulated: when one learns two closely related languages at one time it's less efficient because they 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 question asks about the studying of similar languages - the author even claims that they are "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 against learning related languages in parallel - but this claim is not backed up by any scientific research.
This question is about learning similar languages but not necessarily in parallel. It's also not about comparison of learning two similar languages in parallel compared to learning typologically very different languages in parallel.