I’d probably have one effect per separate engine in the “Pro-4” otherwise it would get messy.
Yeah, that's been a bit of a topic of debate for the Rev2: specifically, in the form of complaints that one can only use one effect per split/layer, etc.
Frankly, for many, the effects in the AS-1 are enough.
And, as a point of reference, the Korg Prologue allows one to route one or both parts to each effect (Mod or Delay/Reverb), or to turn off each effect globally.