I was wondering...
When you use your first action to move, do you apply the -1 to your second action?!?

Yes. I've noticed a tendency to play this as if it were Duo Attack, and the -1 applied only to the second attack. But that is incorrect: the -1 is to the damage value for any attack made using this power.

Here's the definition of Multiattack from the 2011PAC:
MULTIATTACK
Once per turn, give this character a power action. It can be given any two actions
as free actions. Resolve the first free action before giving it the second. Modify this
character’s damage value by -1 to a minimum of 1. This ability can’t be countered.

So, it does states that you modify your damage on your second action...

Yes. As well as the first.

can u do a double power action for ziran multi attack on his special planets weapon dealing 4 unvoidable damage ?

I'm not sure what power you are looking at, but Ziran has no Double Power Actions.

He has this ability that could deal 1 unavoidable damage.

There is nothing stoping a character from taking Double Power Actions with Multi-Attack Ability, but you have to have one.

And incase you are getting confused, a Double Power Action is specific on what a character can do, you can not give a character a Double Power action to perform a a regular Power Action twice.

Wow.... this got confusing fast. I thought I knew how Multiattack worked. Let's see.

Give Character a power action.
Modify damage value by -1
Give character free action to do anything they can currently legally do. Resolve.
Give character another free action to do anything they can currently legally do. Resolve.
Action Token, Pushing Damage (Colossal Stamina Push)

Should about sum it up Sesame Street Style, unless I'm wrong.

The bolded only comes into play if you already had a token. Multiattack only gives you one token for both actions!

Well, other than that pushing damage would depend on what their token status was before you gave them the power action, yes.