How do you use one ballast to power two grow lamps at seperate times? say for instance two rooms both on 12/12
If you don't care about the cost then just buy another ballast. What's the reason for wanting to share a ballast between two lights?
Also, why would you have two rooms on 12/12? I agree though. Just get a second ballast if money is no object. Good luck.
Putting one room on and the other off at opposite times would be a great way to control heat imo. As for a way to make that work, I'm pretty sure there is a way, but I can't really help you personally, sorry.
There actually are some benefits here. For starters, when a ballast is starting up, it's at it's least efficient, and it's most likely to fail. Because of this, if you have your ballast or ballasts running 24/7 powering two rooms, it will last longer and use less electricity.
you could prob try and run 2 leads off the ballast just connect another 3 core 1.5 mm or 1.0mm flex cable to the same connections in the ballast , and then put timers on each outgoing leg off the ballast so they come on at different times i dont know if this would burn your timers out though, are you running it through a contactor relay?
The up to 4000 volts of startup voltage for an HPS is not conducive to long life for a typical household timer without a klugey relay/contactor circuit. This is a solution without a problem. One ballast for each bulb.
According to my electrical engineering friend, they are characteristics of both the bulb and the ballast, but you can't really do anything about it in the case of the bulb.
Here is guide to build your own flip flop relay. http://forum.grasscity.com/grow-room-design-setup/507280-how-build-relay-flip-flop.html