PCM and I2S are pretty similar. I2S data or clock is delayed by 1 bit and may or may not have different bit ordering to PCM (this is from memory of looking at some datasheets 5 years ago).
As to "muxing" to create a single audio file with both MIC and SPKR channels, that depends on the audio format. If you are saving as an uncompressed WAV, then the channels are interleaved at the sample level (i.e. channel 1 sample, channel 2 sample, channel 1 sample, etc.).
Unless you meant muxing the hardware lines; it's easier just to use two synchronous serial modules. If you used a single serial module and clocked it at twice the bitrate (with a hardware MUX enabled at the bitrate) you'd get both channels, both the data would be bit-interleaved, and require more processing (you could use a LUT to get the two channel's nibbles out of each databyte quickly).