First of all:
When a canon flash fires in ETTL it sends out a pre-flash of about 1/32 power. This signal is then bounced back interpreted and the flash calculates the correct exposure setting and then fires.

First question: why are flashes made so that we don't know what this setting was.
Why can I not see what power it just flashed on ( 1/8 1/4 1/2 ??)
My usually procedure is to adjust the FEB up or down, but if I wanted to switch the flash to manual mode it would be nice to know what this setting was.
Ok so I can live with it.

My real question is:

When I use off camera flash in ETTL, say the on camera flash is A and there is a single off camera flash B.
On camera is the master A and B is the slave.
Ok fire in ETTL. Pre-flash. Instantly calculates correct exposure just like if there was only one on camera flash, and then both flashes fire.

Question: When the info returns from the pre-flash (assuming the balance is 1:1) does 1/2 of the calculated exposure go to each flash or does each get a full dose.
If they both get a full dose, why am I not constantly overexposed when using A:B flash?