MOTM

>You need:
>
>a) a polyphonic MIDI-CV converter. The MOTM-650 has 4 'voices' and can be
>daisy-chained with another one for a total of 8.

or an polyphonic gate/cv keyboard like a (4-note) Roland M-184 or 
(2-note) Synth.com QKB15S.

>b) you need multiple 'sets of voice modules'. The most 
>simple/cheapest would be:
>
>1 MOTM-310 VCO
>1 MOTM-490 VCF
>1 MOTM-800 EG (same envelope for VCF and VCA)
>1/2 of a MOTM-190 VCA

or you could cheat like the Poly-800 and have just one filter :-)

another much cheaper option is to sample some notes of the sound you 
want to make polyphonic, and then play it from your sampler. heresy, 
but it works pretty well if you take the time to multisample properly.