Pope Innocent III ordered the construction of double piscinas, I believe, in 1216, to ensure that the water of the handwashing was kept separate from the water/wine of the ablutions before Communion. By the end of the century the regulation seems to have been regarded as otiose.
This is a simple example, without a double canopy, at Thriplow – but what are the ridges between the drains? Pevsner is silent.
A Bishop's Blog 
For the soap?