how do i which payment to a different card

I understand from your question that you wish to direct specific payment (in the iTunes and App Stores?) to different cards.

The payment system is automated:

How App Store and iTunes Store purchases are billed - https://support.apple.com/HT201359

"Which payment method is charged?

When you buy items or pay for subscriptions, Apple attempts to charge your payment methods in this order:

1. Apple attempts to use your Apple ID balance to cover the full amount.

2. If you don't have Apple ID balance or you don't have enough to cover the full amount, Apple attempts to charge your primary payment method. The primary payment method is at the top of the list in Settings on your iPhone. You can control which payment method is at the top of the list.

3. If there's a problem with the primary payment method, or if the primary payment method can't be charged for the full remaining amount, Apple attempts to charge your other payment methods in order from top to bottom. If none of them can be charged, Apple may try again.

Unfortunately you can't tell it to pay one thing with one card and another with a different one. The closest you can get is to move another card up to the primary payment method when you want that card to be used. Either that or use a different Apple ID for the various cards and their subscriptions/payments, but that has its own nightmare of complications and is best avoided.