How Does the expert belt work?

[Chocolatebear]

if the attack says it does 30 damage to each of their pokemon does it mean that it will do 50 to each of them instead?

 

[baby mario]

Have another read of Expert Belt.

It only increases damage done to the ACTIVE Pokemon.

 

[Rogue Archetype]

if the attack says it does 30 damage to each of their pokemon does it mean that it will do 50 to each of them instead?

I thought the same thing when I first saw the card.

I was like "TYRANITAR FOR THE WIN !!! "

But... sadly... it only does that extra 20 to the defending pokemon.
Their bench pokemon don't get the additional 20 damage

 

[BJJ763]

if the attack says it does 30 damage to each of their pokemon does it mean that it will do 50 to each of them instead?

The attack sounds like it is worded for 2 on 2 play, which wording was big in the EX Series, in which case both Active would take 50.