Hello , if Scarn uses Flail against two adjacent targets, and gets one Surge, plus Hits. Can he use that Surge against Target 1 to Mend 3 and then again against Target 2 to Mend again (Mend 3 x2)

No. There is only 1 attack made, surges are never double counted.

For example, monster flails and rolls 3 hearts and a surge, let's imagine he has surge: +2 , surge: curse, surge: mend 2.

One hero rolls 2 shields, one rolls 3.

Monster has 3 options:

Option 1: spend surge on curse. Hero A will be dealt 1 damage and be cursed, hero B will be dealt 0 and not be cursed.

Option 2: spend surge on +2. Hero A will be dealt 3 damage, hero B will be dealt 2.

Option 3: spend surge on mend 2, healing 2. Hero A will be dealt 1 damage, hero B will be dealt 0.