I need to stack two 432mhz quads, not in the traditional vertical plane,
but horizontal. This brings me to two questions... (1) What distance
apart? 1/2 wave length or 1 wave length or ?? (2) Do I need to make
that measurement from boom center line to boom center line OR would it
be end of element to end of element?

Another one of the all depends questions. If you want a clean patern it is
one spacing, maximum gain is another.
Forget stacking quads at 432 and go with a longer length yagi if you want
the maximum gain.
Quads are good for maybe 6 meers and lower. By the time you get to 2 meters
it is beter to go with a long yagi as the aditional gain of quads start to
fall off compaired to yagies of the same boom length.