Can only select first channel of multichannel soundcard

I installed a terratec Phase28 Soundcard under Windows XP Home.
According to the manual I should be able to choose from four
stereo-channels in audio-applications but only one (the first) is
shown in all menues. Tried Samplitude 6.0 and Radioclient On Air,
which has a seperate device-mapping program.

What could I try next?

Detail: In the soundcard-choose-menues there should be
Phase 28 1/2 (first stereo channel, first two mono channels),
Phase 28 3/4,
Phase 28 5/6,
Phase 28 7/8

But only "Phase 28" is shown, nothing about channels and sound is
always coming from mono-channels 1/2