[red-beard]

A few outsdide suggestions

1. Create a Master bedroom only zone. The easy way is to add a mini-split type unit to the master. You can leave the existing ducting in place.
2. Continuing, make the upstairs the main zone for the second circuit. Move the thermostat to the second floor
3. You said all of the equipment is on its last legs. Replace it. But replace with TRUE varible speed technology. NOT two-speed, but a system using varible frequency drives. These systems can speed up and slow down as needed to give exactly the amount of cooling needed, saving a lot of energy. This said, the pricing on the units is higher, but the price difference between a standard single speed replacement and the VSD units will pay off in a few years, especially with California energy prices.
4. On the VSD Air Con units, don't by the "top" model, buy the next level down. The difference between 25 SEER and 23 SEER is not enough to make a difference.
5. Consider a "geo-thermal" heat pump. These reject the heat energy into the ground instead of the air. The heat sink is more consistent and the cooling cost are much lower in the summer and heating much lower in the winter. These can be very pricey, so keep your eyes open if considering.