Two independent current sources with one opamp?

A current mirror might work:

You can use a current mirror with 2 NPN's in the configuration shown to mirror a current. This ensures that the Load R4 has the same current as the current running through R2.

^{simulate this circuit – Schematic created using CircuitLab}

This circuit shows what is possible (and is unfinished because I have not sized the Vcc's or the resistor). But you could make an adjustable current mirror with this circuit. The idea is the opamp adjusts M1 to create the same voltage on the negative terminal as the positive terminal, so if you have 1V on the positive terminal there will be 1V on R2 (and the current $ i = frac{v}{2R}$). If you use a mosfet for M1, make sure the gate voltage is high enough to turn on M1, this means you'd need to have a higher Vcc on the op amp to turn on the gate than the Vcc of M1 or use an NPN with an appropriately sized resistor). These circuits are hard to compensate to avoid ringing, so this circuit will need some filtering on the feedback path.

^{simulate this circuit}

As Andy states you will need an op-amp per current source. One can simply not direct a specific current in more than one direction.