2 My Book Live's 3 TB each, mapping as 1 single Drive?

Looking to do something similar to Raid 0 between 2 3TB My Book lives so as to create 1 6TB my book live.

Of course i would like this to be scalable so as if i decide to expand to a 9TB setup or even a 12+ TB setup.

Is there any way to do this or am i out of luck and going to have to just use each drive seperately?

This is not possible.  Also you cannot map the whole drive, the mapping can be done per shared folders.