It depends on how you have the cards listed in your Collection.
If you have 2 in the same listing (with the blue "2" to the left), "Add Another to FS/T" will create a new listing with 1 in FS/T, but you'll still have 2 in Collection, while "Add to FS/T List" will move both to FS/T. However, if you have two separate listings "1" and "1" both in your Collection, you can "Add to FS/T List" on one of them to leave you with one in each category.
Alternatively, by clicking on the blue "2" number, you can change the "Listed in" value to For Sale/Trade, then change "Quantity" to 1. That will move one of the two to FS/T & leave one in Collection, so you don't have to go back and change the number in Collection - if you did it the way described above.
Simple enough, eh?