As I can see on their official website, the only way to acquire magazines is with the help of an account. All the purchases are attached to the account you use. Simply log-in on one device and buy the magazine, and then log-in with the same account on the other device and you should have the possibility to read it there as well. There is no need to buy it twice. Technically, since you purchased the magazine, you have the right to do whatever you want with it.