2000 MP to execute 2 skills. Very power inefficient.
180 seconds to execute 2 actions at once. Just who is he fighting, the DMV staff of Zootopia?
TL says it's about 80% correct, so I take it this is part of the remaining 20%.
Most likely the correct meaning would be something along the following lines:
The hero invokes / executes the double action skill for 2000 MP.
For a duration of 180 seconds the hero can execute 2 actions at once.
So it is probably a skill that allows the hero to use different magic spells, skills or / and normal attacks twice as often as normal.
Like if he can usually shoot one fireball spell at a time from each hand, then he can now shoot 2 different fireball spells from that hand at once.
Therefore it basically raises his fighting power to 2 times his usual power.
That is not inefficient at all if you really need to raise your fighting power.
The MC most likely just used it to make sure he can take down the yakuza before they can fire a single bullet at Leo.