When a female places a tampon in both her vagina and her butt. Double string swag can also occur if a tampon applicator is mis manufactored with two tampon absorbers in the same applicator, causing a girl to insert two strings into her vagina.
Guy: Babe! Why is there a string hanging out of your butt?
Girl: I'm on my period and I'm too based for one tampon. I got that double string swag. Wooot!