The SSD1306 data sheet says that even in 3-wire-SPI mode /CS is used as chip-select. So this should work as expected.
If you module really has only the SCLK and SDIN pins: use an OR gate in each of the SCLK lines to your displays. Only the display where the second input to the OR is 0 gets a clock, for the other one it stays 1 all the time. The OR allows you to use digital signals all the time, I guess the analog mux might not be fast enough for your purposes.
The thing with the OR gates does the trick.
Two OR for the two Wires ( SCL and MOSI ) for each Display
And then one NOT and a 1 BIT wide CONTROL REGISTER. Nice thing.
Works really well.
Ill post later some schematics and screenshots.
To give proper credit: I found this idea in an AppNote from Analog Devices, since their accelerometers use the CS signal to switch between I2C and SPI, so it doesn't disable the SPI interface.