Hi Sam,
How about using TieColum.tie-configuration? Together with Tie.details.height-limit you can avoid the collision.
I don't know how to shorten the Ties, but I used Kieren's whiteout idea to make them finish them in the Stem. If someone knows how to actually shorten them I'd be happy to know!
Snippet below, hope you find it useful,
Stéfano
\version "2.19.81"
\language "english"
left = \relative c {
\time 2/2
<<
{
s4 <a' c>2
\override Tie.details.height-limit = 0.6
\override Tie.layer = -1
q4~ |
\override TieColumn.tie-configuration = #`((4 . ,UP) (6 . ,UP))
q q2 <fs a>4 |
}
\\
{
<f f'>2-> <e e'>-> |
\once \override Stem.layer = 0
\once \override Stem.whiteout = 2
<ef ef'>-> <d d'>-> |
}
>>
}