I was able to have the desired behavior without using drag and drop...
Basically, set up 2 global variables (on -1 as start value), to catch the UID of each chess piece touched on the swipe movement.
On the "Input" group, instead of "on touched chess" event, substitute for:
On any touch start > set variable1 to chess.uid
on any touch end > set variable2 to chess.uid
Then, on the SELECT_2 state:
on "SELECT_2" transfer logic > main:set chess1_uid to variable1
main:set clickedchess_uid to variable2
And finally, whenever you set the main.clickedchess_uid to -1, you also need to set both global variables to -1.