The Shimano EW-JC130 Y-connector cable is used to connect two shifters to a Junction box or Systems Informations Display while using only one e-Tube port. It is generally used with the EW-RS910 junction - mainly because that specific junction only has two e-Tube ports.
The EW-JC130 comes in three different sizes to fit different handlebars and placement of Junction A EW-RS910, EW-JC200 or the EW-WU111 Wireless Unit. Refer to the image below for an overview of the sizes.
Note that L1 corresponds to the Left shifter and L3 corresponds to the EW-RS910 handlebar junction.