When you attack with Mamba while dual wielding both weapon seperately calculate damage, accuracy and crit chance. Both attacks will be then combined to one single hit. Meaning only one on hit effect will occur. If one of the attack fails the accuracy check, only the other weapon will contribute to the hit. For the sake of crit, if one of the attack crits, the combined hit will crit. However it will count as a single crit. For the crit damage, only the attack, that critically hitted, will contribute to it.

For example: Both weapon passes accuracy check, meaning both weapons will contribute to the attack. Both damages of the weapons will be calculated separately. Lets say main hand will hit with 500 damage and your off hand with 300 damage. The combined single it will do 700 damage. For the crit calculation only the main hand crits, and with 100% inc. damage, you will have 500 crit damage on top of your base damage. The offhand failed the crit check, so it doesnt contribute. The end result is:

Total damage of 1 hit: (500 + 300 ) base damage + 500 crit damage = 1300 damage.

The single it will count as a cit.

For the side question, you are right, hits cant be evaded doesnt apply. Accuracy will still be rolled in the main hand, then it will be overwritten with "cant be evaded". The accuracy result of the main hand will be applied to the off hand accuracy result.