Others disagree, buy my choice is to buy a boxed version with a disc in it. You get a license to run on two computers and one can be a PC the other an Apple. If you download, it can be only one or the other.
Once you pay for either, all you get is a serial number. Each time you start the trial, it should give you an option to provide the serial number or continue the trial.
One warning, any final output done in the trial will have a watermark. Be sure you keep any trial projects so you can remake the output without the watermark.
Each computer will have its own trial. They are not related. You can try both computers for 30 days.
If you like the software and have bought a license you can install PE14 on the two computers (with one serial nr.)