First time Voices user

[Br. Samuel Springuel]

So, I'm trying out voices for the first time, switching a piece I had 
written using chords for single staff polyphony to using voices.  In the 
process I have run into a couple of things I don't understand:

1) Some notes that don't collide (to my eye), are getting shifted as if 
they did.  When the same pair of notes are set as a chord, there is no 
shift.  Clearly there's a difference between what lilypond considers a 
collision between notes in separate voices and within a chord.  Can some 
one explain that to me?

2) When simultaneous notes from separate voices have the same 
articulation, the articulation is printed twice (once for each note). 
Obviously, this is desirable behavior if the notes are articulated 
differently, but is there some way for simultaneous identical 
articulations to be printed only once?  The identical articulations tend 
to make the notes look crowded.


Below is a snippet which creates two staffs showing examples of my 
confusions: the first is my test using voices, the second is the same 
music typeset using chords.

%%%%%%%%
\version "2.18.2"
\language "english"

mel = {bflat'2 bflat'4^- a'^- a'^-}
harm = {g'2 g'4^- c'^- f'^-}
chorded = {<g' bflat'>2 <g' bflat'>4^- <c' a'>^- <f' a'>^-}
global = { \key f \major
        \set Score.timing = ##f
        \omit Stem }

\new Staff \with {
        \remove "Time_signature_engraver"
}
<<
        \global
        \new Voice = "melody" {\global \mel}
        \new Voice = "harmony" {\global \harm}
>>

\new Staff \with {
        \remove "Time_signature_engraver"
}
{
        \global
        \chorded
}

--
✝✝✝✝✝✝✝✝✝✝✝✝✝✝✝✝✝✝✝✝✝✝✝✝✝
Br. Samuel, OSB
(R. Padraic Springuel)

PAX ☧ ΧΡΙΣΤΟΣ