What are TWICS
It is a credential U.S. mariners are being required to have. It is a card the size of a credit card which includes your picture and an embedded chip which contains information about you. People who do business or are members of all U.S. transportation branches (rail, air, shipping, dock workers, etc.) will be required to have one.
The MMD is a card also that looks like a driver's license and give information about you and your rating/qualifications.
The government is trying to combine the two as they both serve the same function, especially in the shipping world.