Yep, getting two small MCUs is likely cheaper and getting two ADCs in the same MCU synchronized so they sample at the same time is also easy...but getting the same thing to happen in ADCs spread among different MCUs is trickier and less reliable, especially if they are running different code or the same code that is not always synced.
However, do you really need them to "simultaneously sample" for your audio sampling application? After all, even a multiplexed microcontroller ADC can jump between channels at 1MHz whereas the bandwidth of audio is only 20kHz. If you're sequentially sampling 4 channels at 1MHz when the signal bandwidth in each channel is only 20kHz, all 4 samples are pretty much simultaneous already since there's going to be almost no change in a 20kHz signal in the 1us it takes to jump to the next channel (3us for the time between the time the first channel is sampled and the 4rth channel is sampled).