I also would love some explanation on this!
So far I have heard a rumor that the first x2 double drop only lasts for 20 rounds.
I had already bought my 4x50diamond stamina buys for the day and the message I got on my stamina screen was " Buy 6 more Stamina Refills to activate a x2 Campaign Drops!"
I assume this is meant to apply from your first daily stamina buys and so it would trigger after you second 100 diamond buy or after 400(50x4 100x2) total diamonds spent, if not then it is implying I would need to buy 6 more 100 diamond buys which would require a total of 800(50x4 and 6x100) diamonds spent on stamina to have a x2 double drop apply. I am still unsure on how long these double drops would last.
I am VIP 14 and I have 15 daily stamina buys. I assume the x3, x4 drops are reserved for the highest VIP levels, and not even sure If I would be able to access them at VIP 14.
It would have been nice for them to include details about it in the patch. I, for one, will not be testing it out until we receive a level cap AND a promotion increase. Hopefully by then someone else will just have the answers for me.