Pipeline look like 
gst-launch-1.0 filesrc location=/filepath ! wavparse ! output-selector name=os os. ! alsasink async=true os. ! alsasink async=false

I am trying this pipeline through code. First I set one of alsasink async=true other as async=false to preroll. After that I am setting both the sinks async=false.
We output-selector is connected to prerolled sink playback is fine. But when it is connected to othersink (i.e at the beginning or once i change active pad to this un prerolled sink) it is not playing and giving below error: 

basesink gstbasesink.c:2198:gst_base_sink_do_preroll:<sink2>[00m prerolling object 0xb2403370
basesink gstbasesink.c:2217:gst_base_sink_do_preroll:<sink2>[00m preroll buffer 0:00:09.800000000
audiobasesink gstaudiobasesink.c:1104:gst_audio_base_sink_preroll:<sink2>[00m ringbuffer in wrong state
audiobasesink gstaudiobasesink.c:1105:gst_audio_base_sink_preroll:<sink2>[00m error: sink not negotiated.

But when I connected to same with mp3 playback by placing output-selector in between parser and decoder (... mpegaudioparse ! output-selector name=os os.! mad ! alsasink os. ! mad ! alsasink)  It works fine for any number of time switching active-pad of output-selector.
But if you place output-selector after decoder i.e directly to sink. It is giving same error as above.