Two Arduino Pro Micro joystick connected to a PC

I have modified 2 SEGA joysticks and connected each of them to an Arduino Pro Micro to make them work with my computer. When I connect one of them to the computer they work fine.
But when I connect both of them at the same time, the one which connect later is not recognised.
In the control panel the first one appears as Arduino Micro, and the second appears as an Unknown Device.
I am using Windows 7 on my computer. And using the Joystick library and the same code on both Arduinos. On one of them I changed the 'JOYSTICK_REPORT_ID' from 0x03 to 0x04, hoping it will help, but i get the same results.
How could I make both Joysticks work at the same time?
I was considering using just one arduino, connect the buttons of both joys to that one and use the feature of the library that it can add two joysticks to the devices.
But i would like to avoid that solution and modify the software somehow to make it work.
Thanks in advance for any help,