<VV> more brakes: question

[Stephen Upham]

i had the dual m/c installed on my '65 (Clarks).   I have personally  
replaced all of the shoes, springs, wheel cylinders, and hoses on the  
front and rear in addition to adjusting the shoes and replacing the  
brake fluid and bleeding the lines with a Mity Vac.  As with most  
things, the more you learn, the more you realize you really don't know  
that much.  Time for a new lesson.  I thought that the idea of the  
dual m/c was that one reservoir was operating the front brakes and the  
other was operating the rear brakes.  If you had a leaking cylinder,  
as I at one time experienced, then I thought that even if that led to  
a complete loss of fluid from that side of the m/c, that the other  
side would brake the other two wheels.  What am I missing here?
The reason that I'm asking is that I have seen several posts that  
related incidents where there was a leaking wheel cylinder that lead  
to a nearly complete loss of breaking on all four wheels

Stephen Upham