The signal that is sent by an RCA connector is just a voltage signal with varying frequency and amplitude. Since a splitter basically places both devices in parallel with the source, the voltage to each device remains the same (Ohm's law) so each device will get the same signal at the same amplitude (voltage) and the same frequency. The only difference is that the source will experience twice the current draw (assuming that the connected devices are identical) and since Line level outs are so low power, it doesn't really have much effect.
The reason why you can have a degradation in signal on very long cable runs is because the resistance of the cable itself will drop the voltage. The long wire basically acts like a resistor in series with the receiver and speaker. A splitter has such low resistance, and is at such a low power, that the voltage drop at the end of a splitter is pretty much negligable.