Encoding 10-bit data on 2 8-bit analog channels
Solutions:


Use a micro with more PWM bits (many have 16-32 bit PWM)
Use a 10 bit separate DAC to generate your 10 bit value.
Dither the 8-bit PWM in software (by adjusting the PWM value on each cycle) to increase resolution.
Use 4x 8-bit channels in parallel, averaged with resistors.