If a character can somehow get Blaster +20 and twin attack, can he do two attacks when he does his Blaster +20? I know it says replaces attacks initially, but it also allows you to make an attack at +12 for 20 dmg. Twin attack states "Whenever this character attacks, it makes 1 extra attack against the same target"

Here is the glossary definition for Blaster +20:

Instead of making its normal attack or attacks, a character using this special ability can make one attack against an enemy in line of sight at the specified Attack bonus, dealing 20 damage.

Yes.