The Maestro Control Center can only connect to a single Maestro at a time. This means that since the Maestro with the most number of channels we have is a 24-channel Maestro, the Maestro Control Center can control at most 24 servos.
However, there are a couple things you can do to achieve control of 30 servos with a system that uses a 12-channel Maestro and 18-channel Maestro. One way is to plug each Maestro to your computer with its own USB cable, open up two instances of the Maestro Control Center, and use the drop-down box in one instance to connect to the 12-channel Maestro and in the other instance to connect to the 18-channel Maestro. Another way is to plug both Maestros into your computer with their own USB cables, open up a single instance of the Maestro Control Center, and use the drop-down box to switch between the 12 and 18-channel Maestros.
That daisy-chaining phrase describes a way to connect multiple Maestros to a computer in order to control servos via a serial interface (which is different from solely using the Maestro Control Center). You can learn more about how to make daisy-chain connections under the "Daisy Chaining" section of the Maestro's user's guide, which can be found under the "Resources" tab of its product page. If something in that section is confusing, please specifically describe what it is, and I would be happy to help clarify.