Connecting six stepper motors to Arduino Uno
To answer your question, probably not.  But don't let that stop you.

Connecting two identical shields to a single Uno isn't going to work unless they explicitly support it, because the pins will clash.

So can you find yourself a CNC shield that is 'stand alone', if it had either an I2C or SPI bus interface then that would be even better?

Or can you find 6 stepper motor controllers that can be controlled from a smaller number of pin (or bus)?

Or can you find yourself an alternative processor with more grunt or more pins, such as a Mega or ESP8266 or ESP32?

Do a bit of research, get a few possible solutions and then ask on here for advice on why not to choose them.  Of course the more details and links you can provide the better informed peoples advice will be.