Sorry for not getting a chance to come back to this til now..
My basic use case is that like @plainsane, I have ample water pressure and my system could easily run 2 or 3 zones at once. It is not really a necessity other than it would be nice to reduce the total amount of time it takes for the schedule to run. Right now it takes about 3 hours, but if I could run two or 3 zones at once, the schedule would be complete in 1 hour or so and also be complete before the sun really hits the lawn.
I hadn't thought about piggy backing multiple valves into a single terminal, but I think I would still like the flexibility of being able to run them separate also.
I too also run a master valve, so if the limit is 2 solenoids at one time due to hardware power supply, that answers that! Maybe if it became a possibility in software, I could look into adding a relay to get more solenoid power.