why does the Farmer's walk assist your grip so well?
could you not get the same effect without the use of your legs ?
Therefore it would be more of a static hold with dumbells.

My grip sucks BTW.

However, here is a tips that I have read about and agree with.

Try training your pinky finger specifically.
Ever notice how in a static hold of DBs the grip will peel away from the outside to the inside ?