1: Open ATDpack and extract the files of the character you want to make "2-fisted"

2: Open "XXXXX.att" using notepad and look for a line that says TWO_FISTED.

3: Change the value to 1.

3a: Maybe also look for RIGHT_HANDED and change the value to 0.00000 if you want.

4: use ATDpack to zip the files back up and replace the old ATD file with the new one (don't forget to make a backup).

4a: Or if you use xtended mode, set settings to "Prefer unpackaged content"

5: try it out.

If you're still not sure, look for Oren Yona's "Assassin" character, and compare and contrast.