Interrupts on the 6602 can be used.
Attach an ISR to this interrupt, and update your analog outputs.
Generate a clock with the 6602, if you have 50% dutycycle it can be done with 1 channel; if not you can use 2 counter-channels (e.g. 1st to generate the clock, and a 2nd as a 'retriggerable single pulse' generating the interrupt).
The RLPM of the 6602 is not complete about interrupts, but I posted the necessary information.