This is a follow-up, and hopefully an improvement, to my question on German and Yiddish that was migrated to German from Linguistics: https://german.stackexchange.com/questions/18895/learning-german-and-yiddish-at-the-same-time (See also Why was this migrated? )
What is known about the process of an adult learning two closely related second languages?
Below are some more specific aspects of this question I hope can be addressed, but please feel free to discuss the general question from a broad perspective. Where information on these questions is unavailable, make reasonable inferences from what is known.
How do learning outcomes differ from those of adults studying unrelated languages? How does the extent of interference between the two languages compare, for instance, with interference caused by one's first language in a more typical language-learning situation?
Can it be expected that there is a significant advantage to learning the two languages sequentially rather than simultaneously? If so, how big is the advantage? (I know it's hard to quantify these things, but for example, would the advantage be significant enough to make it worthwhile for a typical learner to delay learning one of the languages?)
More generally, what strategies specific to this situation have proved, or would likely prove, useful for learners?