The existing information says that you can only have 1 of them active AT THE SAME TIME, however there are several issues with this:
a) you can only see it if you scroll down so, many people only realise it when they go for the 2nd free calendar and saw that message about discarding the previous one.
b) it is not clear if it is possible to run one of the calendars for X time and then switch to the other. This issue is even more relevant if we verify that their duration/validity is greater than 31 days (more specifically, 45), which makes it possible to collect 15 days from one calendar and then 30 days from the other (this is what I'm understanding from it).
c) if the intended idea was to force people to choose between the 2 bonus calendars and discard the other, then that should be done in a total different way (one pack on the store with a selection inside it).
I also appreciate that devs clarify all this cause the current status can lead people to several kind of mistakes.
Thanks.