\partcombine with chordal instruments

[Richard Shann]

I've been asked to typeset two guitar parts in an orchestral score. For
the full score the parts could be combined on one staff for which
\partcombine is good except where there are chords involved:


\version "2.18.0"
guitarI = {
         <e' a' e''>2. r4
         <e' a' e''>2. 
}
guitarII = {
         <e' a' e''>2. r4
         <e' b' e''>2. 
}
\score {
\new Staff  << 
 \partcombine \guitarI  \guitarII  
 >>
}

The first chord here looks like it has too many dots and the doubled
stems are not desirable here I guess. Does anyone know if there is an
agreed standard for engraving this and how to achieve it in LilyPond?
The parts should be capable of being included in a \score {} block as
separate parts, so solutions need to leave the actual definitions of the
music of the parts intact (more or less).

Any help with this would be much appreciated.

Richard