I am a non-EU citizen on a spouse temporary visa for Germany. I have been admitted into a Masters course in Italy and need to apply for my visa to start the semester online in September. I won't be physically moving to Italy until February so need to maintain my German visa until then at least. Simply applying and getting the student visa for Italy won't automatically cancel my German visa, will it?
No, through the application itself your German residence permit will not become invalid.
Only when you permanently leave the country (i.e. you deregister your residence in Germany and leave) does the residence permit become invalid.
The corresponding paragraph is § 51 (1)(6) AufthG
(1) The residence title shall expire in the following cases:
6. if the foreigner leaves the federal territory for a reason which is not of a temporary nature,