How do voltage and resistance work for two 5v servos when using an external 12v power supply?
Because the supplies of both servos are connected in parallel, they both see the same voltage. Hence there is no "pool", both are being supplied the same 5V.

There is a regulator on the Arduino which reduces the 12V input down to 5V. It does so by burning off the other [7V times however much current is being used] as heat. Since you are supplying it with 1.5A, the most it can supply is in turn 1.5A.