basically you are right.
But there is signal processing algorithm to track two dongles sampling/oscillator error separately and compensate them separately. Thus we can get them synchronized finally after signal processing algorithm.
What I need actually is that how to get two streams of sample from two dongles.
Actually I notice that in dump1090 it put rtlsdr_read_async in a new created background thread, then there is a possibility that I create two thread to operate two dongles. But in this method, there should be some mechanisms to synchronize two threads. It is complicated.