Networking using a cat5 Lan coupler

I want to share broadband on two different machines at my home.
For that I am using ‘cat5 coupler’ to divide the cable from modem to two different computer. One of the cable from coupler is 30 meters long and other is 15 meter long.
Using only one cable with coupler works fine but as I insert second cable connector to the female port of coupler, ethernet light on modem gets off.
Please let me know, where I am wrong.

Thanks in advance.

atul