Ok, I got it to work.
The SoundBlaster PCI 128 (Ens1371) does support 2 streams, via 2 x DAC's.
I found that I could initiate 2 concurrent streams like so:-
aplay -D hw:0,0 test1.wav &
aplay -D hw:0,1 test2.wav
That is the 1st stream to DAC 1 (hw:0,0) and the 2nd to DAC 2 (hw:0,1). I then found that the Alsa plugin for XMMS allows configuration for either DAC. I set XMMS to use DAC 1 (it used DAC 2 by default). Now I have conccurrent sounds from XMMS and my desktop/applications! w00t!
Thanks for your help.