Multi-plex 8 BLDC motors using shift registers?
Question: Can I drive them all from one circuit with a shift registers
  or with a multi-plexer.


Yes


  If so, how?


Use MOSFETs to connect individual motors (complete with reverse polarity protection diode) to a "bus" voltage. The "bus" is the PWM switching power source. The 8 mosfets need 8 control signals and if you are low on GPIO then a shift register would work fine.

You might even be able to use one of these: -



It's got four high side and four low side mosfet drivers and runs from SPI control. I'm also sure you can get a MOSFET equivalent of the ULN2003 octal open-collector driver.

There's also the TPIC6C596 from TI: -



It's got a serial input line with 8x 250mA rated MOSFETs.