Double card bonus + Double card for 250 diamonds issue

[Janus]

The diamonds increase the number of cards you receive with the drop, not the frequency of the drop. So the chance of card dropping does not change, but when they do drop, you will receive 2x the amount of cards you normally would have.

So for example, I am currently recieving 8 cards per card drop at the moment:
a. Inferno 1 (1+1) 2
b. 250 Diamonds for 2x card drop (2*2) 4
c. Double card drops bonus (4*2) 8

http://gyazo.com/6cf11c16dfe441b1af9787ba7263a15d
http://gyazo.com/150ff4bc3161337dcc94242b5c589646
http://gyazo.com/0ed9262aa1f24376b915298295dfc13f
http://gyazo.com/242f7dc5323ade713822d72f50824832

So you can see the bonuses do stack, but it's not for twice the chance of receiving cards, but for twice the cards (amount) when they do drop.