Edit: This is now cross-posted to linguistics.
Studies have shown that learning more than one language helps people of all ages learn each one of the languages. For a nice mainstream media piece on this, see this New York Times opinion article.
The studies have focused on distinct languages, such as English and Spanish, but my question is whether there are relevant studies with regard to learning Mandarin and an additional Chinese dialect.
To summarize the aforelinked opinion piece, the previous paradigm was the following: exposure to multiple languages presents us with cognitive challenges that interfere with our language learning.
The new paradigm is that these cognitive challenges, rather than interfering with our language learning, force us to reconcile them in ways that are beneficial to learning each of the languages.
Anecdotally, I have certainly found this to be the case as I learned Mandarin and Nanjing's dialect while living in the city of Nanjing for about 15 months. On an even smaller scale, I found my Spanish quickly improving while visiting Galicia, a region of Spain where a language closer to Portuguese is spoken.
Re-stating my question succinctly: are there studies on the learning outcomes for non-native Chinese speakers who learn Mandarin in conjunction with an additional Chinese dialect?
If not, I would also welcome general feedback or comments concerning my question. Thanks!