Hi Alex,
> If I only connect one camera the ASiair will recognize it for a short period and then freeze!
> Turning the ASiair on with nothing connected to the inputs (usb), no problem...
These two clues sound to be almost be like a power starvation problem. If so, you might also see a low power warning in the setup window (the window you see when you press on the button that looks like a WiFi icon).
If you have a DVM, you can open up the Raspberry Pi (ASIAIR) enclosure and measure the 5V pins on GPIO connector (many documentation on the Raspberry Pi GPIO connector on the web). *Be careful* not to short anything. The Raspberry Pi board should see no less than 5 to 5.1 volts.
If you do not have a DVM, I would test using a good wall plug "smart phone charger" that has USB Type A output, to which you can attach a good phone charging cable that has a micro USB connector at the end that plugs into the ASIAIR. Do not use just any data cable, since some of those would be a problem by itself. You need something with heavier gauge wires; ones are usually sold as "charging cable."
I recommend initially testing with a different cable from the one in the ASIAIR box, since that could be the problem in the first place. Be sure the micro USB connector seats well into the Raspberry Pi power connector. You may need to remove the plastic side panel (it pops out, you can find Raspberry Pi video on the internet) to confirm a good connection.
Although a "12 watt" camera charger is a little low, it may be enough to power two cameras. A 15 watt to 18 watt charger should be enough if you keep the USB charging cable short. Try it with 0.5m or shorter cables first, if you have them around.
If a different power supply/power cable works reliably, then your problem is narrowed to 1) insufficient 12V supply to the ZWO 12V-5V adapter, 2) the 12V-5V adapter itself, or 3) the USB-A to micro-USB cable that came with the ASIAIR.
(Be sure to use a 5.5mm/2.1mm connector to supply the 12V to the ASIAIR power adapter (small red brick) -- do not use a 5.5mm/2.5mm connector to feed the ZWO power converter.)
Good luck,
Chen