Split USB Port Into 2 Separate Ports
USB uses very fast signals (480 Mbits/sec at high speed) to transmit data serially. As such, it is very sensitive to transmission-line effects in the cabling. What you have done is created a "stub" connection (whichever host port isn't being used at the moment) that really messes up the data signal because of the signal reflections from both the point where the three sets of wires are joined, as well as the open end of the stub.

Bottom line is, you simply can't do that. You might be able to install a physical DPDT switch to switch between the two host connections, but even that would be questionable.