Some work in this area:

https://github.com/mutability/librtlsdr/tree/async-rearrangements - Rearranging rtlsdr_read_async() so that rtlsdr callbacks are not done from within the libusb callback. This allows you to safely call rtlsdr_* from within the async callback.

https://github.com/mutability/librtlsdr/tree/thread-safety - makes device access threadsafe (in theory), so you can safely call rtlsdr_* from multiple threads (including while rtlsdr_read_async is running)

Oliver