There is a crown reward currently which contains a Double Pet Experience Elixir. This weekend is the Double Pet Experience member bonus.
So, the obvious question to those of us that are new is, do these stack
They don't stack and I don't think we are able to open an elixir while the benefit is running. They're quite precious so KI made a fail-safe on them so we couldn't accidentally use them when we already have the same benefit running (same for the double gardening elixirs from a previous event).
Going from the previous event they pause too if the benefit starts while it activated or while we are offline.
My statements are made due to how it worked with the elixirs (gardening and pet XP) from the previous event and I have not tested this to be sure it still applies. I don't think there would be any reason to change it so it is likely to be correct for these elixirs too