The fusionimage blog describes, how to combine the bandwidth of 2 RTL-SDR dongles in gnuradio. The author describes his method as follows: “I simply took two RTL-SDR dongles at their max. bandwidth of 2.4 MHz, resampled the signals to 4.8 MHz, then shifted the first signal down by 1MHz, the other one 1 MHz up, added them together, divided the combined signal by 2 to lower the noise floor again and finally feed it into a FFT plot”. The result shows the combined spectrum. Moreover, the author states that “this isn’t any useful to decode or demodulate any signals, because of phase and clock misalignment.”

In contrary to the described method, the coherent receiver has the common clock, so that the combined signal could be used to decode or demodulate the signals. Please note, that the both signals have to be synchronised before addition (see https://coherent-receiver.com/products/coherent-receivers/rtl-sdr-extension-card/noise-source for more details).