The Citi Custom Cash and the Citi Double Cash are both no-annual-fee, cashback cards that earn rewards in the form of ThankYou points. Both products are consumer (rather than small-business) credit cards issued by Citibank. The most important difference between the two cards is how you earn rewards:
With the Citi Double Cash card, you'll earn 2x ThankYou points (equivalent to 2% cash back) on all eligible purchases with no special bonus categories or caps on the number of points you can earn. The Citi Custom Cash card earns 5x ThankYou points (equivalent to 5% cash back) in your top eligible spend category each billing cycle, up to the first $500 spent, (1x ThankYou points thereafter). You'll also earn unlimited 1x ThankYou points on all other eligible purchases.
The two dramatically different earning frameworks make these cards a great combo. If you hold both cards, you can take advantage of the Citi Custom Cash card's 5x category and use your Citi Double Cash card to earn 2x on your other purchases.