dd 2 output files simultaneously, is it possible?
 

I didn't see anything in the man pages that would indicate that it could, but it seems like you should be able to.

could I just say:
dd bs=512 if=/dev/sda of=/dev/sdb of=/dev/sdc

or would it be more like:
dd bs=512 if=/dev/sda of=/dev/sdb,/dev/sdc

Or is it just plain impossible?

Why not just try it? Presuming you are willing to overwrite sdb and sdc then it can't really hurt anything. Worst it can do is return an error. In fact, I will bet it does just that.