A USB 3.0 to HDMI adaptor usually has its own GPU in it. If I'm not mistaken, both HDMI connectors need to be connected to the dedicated GPU. If your GPU has other connectors, try getting a displayport to HDMI or DVI-D to HDMI. Don't use dongles like these that don't connect directly to the GPU or you may experience performance issues, or the video won't sync properly between left and right screens.