Using a Canon EOS T3 with Neewer NW-625 speedlight kit with the FC-16 radio trigger. When the trigger remote is connected to the hotshoe on camera, it fires in sync with the shutter perfectly. But when I connect with the 3.5mm to 6.35mm connector cable, it's out of sync. I can't tell if the remote is slower or faster.

So if I also want to use on-camera flash, they are out of sync. Both flashes fire, but not at the same time, apparently. 

Any experience with this issue? Yes, it's an older camera but wtf... 

Do I need to manually sync the flashes? Is there a documented way to do that?