*****

A timer instance (e.g. TIMG8) has a single time base, a function of the (single) LOAD register and (single) ACOND. 

It is possible to run e.g. PWM (compare) on one channel and use the other channel to generate a periodic interrupt, but the PWM and interrupt periods would be the same.

It would be possible to generate a (shorter) interrupt period by adding that period to the CC[1] (modulo LOAD) at the CCU1 event, but that would be limited to less than the PWM period, since that's as high as the CTR counts. For that matter you could do something similar with the PWM, by "pushing" the CC[0] value forward and inverting the compare sense on each CCU0 event; with LOAD=0xFFFF you would have a fairly large range (this used to be common practice back when timers were simpler).

But this would be much more code. It might be simpler to just adopt another timer.