That depends of the spell.
To hit a planeswalker you need to hit a player, and on the resolution of the spell you(the controller of the spell) may choose to apply a redirection effect that deals the damage (full damage) to a planeswalker that player controls.
if a multitarget spell allows only 1 of each target you can hit or the player or only 1 planeswalker.
However, a spell that allows you to select the player several times, you can choose to deal damage to that player, and any number of planeswalkers.
For instance : Arc Trail : cannot hit both target player and his planeswalker, because it has to have different targets.
Fireball : can hit both player and as many pw that palyer has, as long as u have mana to pay the additional costs. Eg: A player has 3 pw out. u must pay 3R+x, to hit all of them. X must be at least 4 (because it will round down, and the damage wil lbe distributed evenly). With a cost of 7R, you will hit each target for 1.
Conclusion: The targets cand be the same , unless the spell says 'another'. For big x spells, moslty you wil; require a lot of mana.
December 22, 2010 5:55 a.m.
Edit : As per instance of the wording you cannot choose the same target twice. My bad.
If it was a card that red : Deal x dmg to target []. Deal x dmg to target [].
you could choose the same target.
so fireball cannot choose the same target twice, and therefore it is not possible to kill more that 1 pw.
December 22, 2010 6:02 a.m.
Thanks, you two. Good answers both. Glad I asked after all.
hamburgers says... Accepted answer #1
Nope! When you do damage to a planeswalker, you are Targeting the PLAYER, then REDIRECTING the damage to the planeswalker. It's an all or nothing, here.
This also means you cannot Fireball a planeswalker owned by a dude with a Ivory Mask
December 22, 2010 5:45 a.m.