Voltage across tape recorders
The author of that article seems to be confused. The caption says:-


  Figure 2. In a matched-impedance system working to the 600 ohm
  standard, connecting two tape machine inputs to the same console
  output would cause a level drop of 6dB, because each of the two
  parallel 600 ohm loads only receives half the signal power.


6dB drop is incorrect, but so is 3dB (half power). It is actually about 3.5dB, ie. slightly less than half power.

We can prove this by examining the equivalent circuits:-







simulate this circuit &ndash; Schematic created using CircuitLab

R1 and R3 represent the output impedance of the source, and the other resistors are the loads. 

In the first case the signal voltage splits equally between source and load resistors, 1/2 across each causing a 6dB drop. However the source's internal voltage has been set 6dB higher to produce a reading of 0dBVU (at 0.5V in this example) on the load's VU meter.

In the second case with two loads in parallel the total load impedance is 300&ohm;. Voltage still splits proportionally to the resistances, now 2/3 across the source and 1/3 across the loads (= 2/3 of 1/2). As the total resistance seen by the source is now 900&ohm; instead of 1200&ohm;, the current it has to deliver increases by 1/3. This current splits equally between the two loads, so each gets 2/3 of the current of the single termination. 

Power = voltage x current, so with 2/3 the voltage and 2/3 the current the power is 4/9 times as much. Decibels are 10xlog10(power ratio), so 4/9 = ~-3.5dB.