testing this configuration using aplay doesn't make any sense. you apparently believe that getting glitch-free audio playback is a trivial task ... it isn't unless you are prepared to use a lot of buffering (which is what happens in a lot of consumer-ish software.

you are also attempting to use ALSA dmix facilities, which is not recommended by anyone for any purpose (distributions that used to use this now use pulseaudio to accomplish multiplexing streams to a single hardware device).

it is certainly possible to play streams to multiple devices at once, but if you expect utilities like aplay to do this correctly on an arbitrary system, you are mistaken. your problems could be in the ALSA dmix layer, or your kernel or your overall system configuration (see, for example, this list of problem areas: http://manual.ardour.org/setting-up-your-system/the-right-computer-system-for-digital-audio/)

you haven't really described what your application area is, but if it involves multiple appplications sharing an audio device and using low latency, then i think most people would suggest you look into JACK, either to use it directly or as a code model (remembering, however, that is is released under the GPL).