Hooking up to the same wires could work, but is not ideal. In the end, this is a philosophical debate, not a technical one. As long as the electrical capacity to hold solenoids open (~300ma per solenoid) is available, the software should not restrict the operator from running multiple zones in parallel. Unfortunately many "dumb" controllers implement this as a feature. A smart controller like Rachio should be able to provide a slick enough UI to make this both easy to setup and obvious to monitor.
This installation is all drip for olives, grapes and landscape plants with different microclimates on terrain ranging from steep to flat and exposure ranging from full sun to partial shade. What we are looking for is the ability to run 2-4 zones concurrently (limited by water pressure) with independently set durations. There are not enough hours in a day to run the zones serially (At an average of 4 hours of drip per zone, 16 zones = 64 hours of run time, resulting in a continuously running pump and 2 dry days between waterings for a given zone). There are other low pressure water features on the property (water slide, fountains) that would ideally be run from the same controller but cannot be since their operation would block watering cycles. The property is located over 100 miles from my home and does not always have someone on site to perform even the simplest manual operations, so all control must be remote.
Our existing system is a homebrew implementation using an Insteon based home controller (Universal Devices ISY-994) to control 32 zones using 4 EZIO8T relays from Smartenit. It works great - when it works. The problem is that 3 of the EZIO8T devices have now suffered hardware failures, forcing me to evaluate options. I would strongly prefer a system that is purpose built for managing drip irrigation to a custom solution that only I can operate. Commercially available solutions such as the Hunter AC-1200 are over $1200 and are geared for ON-SITE, not remote operations and don't offer the great integrations that Rachio does.
So - back to the question - Is there any hope of Rachio offering the ability to run multiple zones in parallel by next spring?