Parallel music expressions on 1 stave with different notehead sizes?

[Rosalie Fay]

How in Lilypond can I engrave two music expressions, \soprano and \alto, in parallel on one stave in such a way that:
1. while they have the same rhythm, the notes share stems and beams
2. while they have the same rest-sequence, only one rest-sequence is engraved
3. the notes of \soprano are engraved in the normal size, and those of \alto are engraved \tiny

I find that
<< \soprano \\ { \tiny \alto } >> shares neither stems, beams nor rests.
<< \soprano { \tiny \alto } >> engraves all the notes \tiny.
\partcombine \soprano { \tiny \alto } engraves all the notes \tiny.

%%% example starts %%%
\version "2.18"
\include "english.ly"

soprano = { g'8 a' b'4 e''8 f'' g''4 }
alto = { e'8 f' g'4 c''8 d'' e''4 }

\score
{
    \new Staff
    <<    { \soprano }
        { \tiny
            \alto
        }
    >>
    \layout { }
} % score
%%% example ends %%%



Rosalie x