blow anything up
lol, you dont understand current 
the power supply COULD supply 20amps max, but the steppers only draw .8A each, so the PSU will give each stepper .8A(at 6v...if you go past recommended voltage, the current draw will increase)
yes it would work, but converting 12v to 6v could generate a lot of heat, maybe better to find a 6v "native" PSU? you could get a second adapter and wire them in parallel to get 6v 2A O.O 
as a quick fix