If I buy the Windows download version from Adobe or Amazon, will that product key activate the second install on a Mac? I assume I'd have to download the trial version for Mac, but Adobe's trial versions can be activated with a valid product key, right?
Will the same scenario work for a Windows upgrade download purchase (I have PSE 7 for Windows).
Any difference if I buy the box/cd version?
If you buy Windows download from Adobe.com, you will only be able to activate the second license on Windows, not on Mac.
You can always download the trial version for mac, but the same product key won't work there.
If you want to buy the cross-platform version, go to your nearest store, and buy the boxed version. You will just need to enter your serial number in your already installed trial build, and PSE will be activated on both Win and Mac.