When you Y an RCA, you're driving two inputs from one source.  This means
the source sees half the impedance it would normally see if you're only
driving one input.  You won't lose voltage because it stays constant (unless
you draw more current than it can supply), however you draw twice as much
current.  In the case of RCA's, it's not a big deal.  The line level input
is a high impedance circuit and there isn't much of a difference.  If you
were to drive 100 inputs off one output, that's a different story.  At some
point, it will cross a threshold of making a difference.

It's similiar to hooking up a stereo and an amplifier.  Just hooking up the
stereo and the amplifier to a car battery still leaves the car battery
supplying 12 volts.  They're in parallel.  However, you're drawing more
current from the battery than if you just hooked up just the head unit.