I have attached a snapshot of differential amplifier with source degeneration equalizer. Here the Text book gain calculation is simple CS amplifier with source degenerated resistance. But the resistance is in parallel with two differential Moses. So how can it be considered as normal degeneration resistance(series with Differential Mos) ?
For differential signal there is a virtual ground at the middle of degeneration. Replace degeneration as equivalent of series connection of the same elements. Simply replace resistor as two series R/2 resistors and capacitor as two series 2C caps. Then you will able to "cut" schematic to two independent degenerated CS amplifiers and consider only one of them.
This is a simple trick for analysis of a differential circuits used in every text book.