A two way power splitter (nominally) splits the power in two equal parts. -3db means the signal is half the power level and this is the loss you should (nominally) see on signals out each port. This also implies that if the voltage input was 1 volt, the voltage on each splitter output would be 0.707 volts. When the signal from one antenna comes back in, a simple power divider will send half the power out the other antenna and half of it goes to the radio.
