Moinmoin,
the method described by hoodoo_ray is exactly what I use to good effect ;-)
This also allows for effects with mono input, but stereo output like some choruses, my beloved MIDI-MuRF, etc.
I connect both P'08 output pairs to the mixer inputs and use the pre (I sometimes use MIDI-MuRF without the original signal) and post (chorus, reverb, ...) aux sends for the effects. The effect returns are then fed back into the mixer.
This requires some knob tweaking and being alert in live situations, but I prefer flexibility over simplicity.
This setup also allows me to feed the P'08 - and other instruments - into the VCF of an old analog synth, enabling high pass filtering, very flexible wahs, etc.
If You want to use a setup like this, try to imagine all Your wishes and needs, then select Your mixer accordingly. My personal choice - Soundcraft M4 - resulted mainly from its 2 pre and 2 post aux facilities.
HTH
Martin