What makes this sex toy popular is the fact that both women bask in the orgasm of each other. The more one tries, the better it feels for her and her partner. It's kinda mutual.
Other versions of this toy include squirters at each of the heads and vibrators.
Lesbian 2: I'm horny and I too miss cock and I hate men.
Lesbian 1 & 2 (together): LET'S USE A DOUBLE DONG!
(15 minutes later): *sighs of relief and orgasm*
Candice: I want to get it tonight, you strap it on!
Babymomma: I strapped it on last night. YOU strap it on!
Candice: Well, we could use the double dong and both get some action...