Here is a VCO system using one IC and a minimum number of parts.

This VCO can have a frequency range of approx. 80 to 10,000 hz by varing the .02uf capacitor. Using a 0 to 5v. input and the capacitor listed, the frequency range is about 300 to 4000 hz with a ramp output of about .5v p-p.

The LM324 used in this circuit is a quad op-amp and since one VCO uses only 1/2 of the IC, each IC can be built into two VCO's.

Construction is not critical and parts can be substituted. Also: One of the input 150k resistors can be replaced with a 500k pot. and a 50k resistor to give a wider frequency range and more flexibility when using multiple VCO's.

I don't know how linear these oscillators are, but for special effects where multiple VCO's are a must, these work quite nicely.

IC1: LM324 (Radio Shack #276-1711)
All capacitors : 9v. or better
All resistors : 1/4 watt or larger.

For two VCOs duplicate the circuit using these pin connections on IC1.