HDTV

I am trying to split a signal that goes to my High Definition TV to a non-HD box which was active in another room. I am using a splitter which had worked earlier. Everytime I connect the two wire from the two receiver, only the HD receiver would work, other receiver could not receive signal.
But if I unplug HD wire from spllitter, other box would work.Am I using wrong splitter,distance we are tallking about is approximately 5 feet. Thanks for help