On the RPi you don't even need an asound.conf, dmixing should already be enabled. To avoid ES/omxplayer locking the card, I had to use these settings:
<string name="AudioCard" value="sysdefault" />
<string name="AudioDevice" value="PCM" />
<string name="OMXAudioDev" value="alsa" />
Any application that tries to set a specific PCM device will break dmixing by locking the card. Applications should be set to default/sysdefault or not set at all. ES was picky about this, but the above combination works both with PA stopped or by having it running and use pasuspender:
# SDL_AUDIODRIVER=alsa pasuspender -- emulationstation
In most cases you should be able to just set ALSA_CARD=0 or specify the card like this in /storage/.config/asound.conf:
defaults.pcm.!card 0
defaults.ctl.!card 0
defaults.pcm.!device 1
defaults.ctl.!device 1
Environment variable takes preference.