Permutation is like shuffling a deck of cards. You need to do it once before you deal the cards, not while dealing each card.
Looks like only one permutation table is supported at a time. So if you need two lists with non-repeating values, you would probably have to use an array. Create a permutation table for letters, copy them in random order into the array one by one in a loop. Then create/reshuffle the table for numbers, copy them into the same array.
There may be an easier way to do this, if you could explain what you are trying to make.