Hi, I've used a few of these 'Dual' hot-shoe TTL cords - they are not 'plug & play'.
Neither Canon nor Nikon make these Dual cords, the reason being that they usually do not behave as you might expect.
If you use a Canon Speedlite in only one Hot-shoe at a time, all is well. If you use two and set them both to M flash using their own buttons/controls, again, all should be well. Both hot-shoes are standard Canon style hot-shoes - the far end has a simple pc out jack as well. You might, that's might, get away with using one of the two set to ETTL, leaving the other in M.
The problem seems to be that the camera system 'expects' to receive voltages & code from a single attached Speedlite and cannot cope with those combined from two attached devices. An exception may apply if 1) both attached Canon Speedlites are identical, 2) both have identical settings on their screens, and 3) they are pre 'II' series Canon Speedlites.
Have I put you off?