Note that you can also register your two Macs with Adobe as well, but that will be 5 devices (2 readers, two macs, and a PC). The advantage of this is that you can then download Adobe content via the Macs.
At least some of the new reading devices that are coming out with Adobe Digital Edition from other providers are going to support Macs as well as Windows. See for example
Cool-er: light, trendy, inexpensive e-book reader. So perhaps Sony will be forced to mend their ways.