polymetric parts

[Werner]

I want to have just 2-3 bars in a piece with different time for two voices.
In snippets I found
Changing time signatures inside a polymetric section using \scaleDurations
but I don't understand that very well.
Why doesn't lilypond just do what I write? it puts the first voice to 2/4 with
the second instead of remaining in 3/4:

one voice (and staff)

...
\bar "||"
\time 3/4
e8 a~ a gis c4 | r8 e, a4 gis8 c~ | c e, a8. gis16 c4 | e,8 a r16 gis c8 des a 
| 
% other voice already in 2/4
gis8 d e4 e, | r2 r4 
\bar "||"
% now all in 2/4
\time 2/4
a4 cis | d d8 cis 

another voice (and staff)

...
\bar "||" 
\time 3/4
r4 r8 c, f8. e16 | gis4 c,8 f~ f e | gis4 c,8 f r16 e gis8~ | gis c, f2 
\bar "||" 
% one voice still in 3/4
\time 2/4 
e4 gis | a a8 gis | a c d f | 
% now all in 2/4
e4 e | f e |

Could somebody explain a solution - but please very easy comprehensible :)

Greeting

Werner