@mkm123 Thanks for reaching out and great question!
@DLane is correct. You can certainly tie in one other zone with an existing one. The only thing we want to point out, for your convenience, is try to choose two zones that have similar attributes and watering needs. That is to say, they are the same vegetation type (cool season grass, shrub, etc) and preferably in similar areas that share the same amount of sunlight, slope, etc.
This will allow your Rachio to water those two zones without having to water a dissimilar zone more often than it needs because the second zone has a bigger need (shorter root depth, more run off, etc). Hopefully that makes sense! Reach out to us on our support site if you need more assistance