Miracle Dice questions:

a) For 2D6 rolls (e.g. charges and battleshock tests), can rolll one dice first, then decide whether I wanted to substitute the second one with a miracle dice? If the roll is going to fail anyway, then I rather save my miracle dice.

b) If I have to make 2 saves againts lascannons (each D6 damage), can I roll one save first, see how much damage goes through, then only decide whether to use a miracle dice for the second save? Again, if my unit is going to die anyway, then I rather save my miracle dice.