I don't believe such an adapter exists, atleast I have never heard of or seen such a creature. They make devices like this for security cameras but they are lower resolution and use RCA inputs. While most monitors will allow 2 or more inputs only 1 is usable at a time. It sounds like you want both inputs displayed at the same time, for that you'd almost need a TV with PIP or split screen support. The only monitos i have seen that support such a feature also had built in hdtv tuners.