Multiple sponsorship is common, and often has representation from both sides.
MR. DURBIN: I'M GOING TO ASK FOR UNANIMOUS CONSENT, BUT I'M ALSO GOING TO MAKE IT CLEAR THAT IF THERE'S ANY MODIFICATION AT A LATER POINT THAT SENATOR ALLEN SUGGESTS, I'D BE GLAD TO WORK WITH HIM ON THIS. I THINK THIS IS OUR UNDERSTANDING BASED ON THE COLLOQUY WE HAD ON THE FLOOR. AND THAT IS THAT THE AMENDMENT 4883, SENATOR ALLEN'S AMENDMENT, BE SHOWN AS THE ALLEN-DURBIN AMENDMENT, AND THAT MY COSPONSORS ON MY AMENDMENT BE ADDED AS COSPONSORS ON THIS AMENDMENT. AT THIS POINT MY AMENDMENT, 4884, SENATOR ALLEN'S AMENDMENT 4883 ARE IDENTICAL.
Even durbin acknowledges it, and allen's amendment is 4883(right before durbin's 4884). This essentially means they worked on it together and submitted it at the same time.