Currently, my main goal with multiple payments is to give customers ability to retry the payment.

1. You checkout and select Stripe as payment method, it is refused, you go to retry payment page, select PayPal as new payment method AND **new payment is created**, previous one is set to failed.
2. You pay with PayPal, the 2nd payment is marked as completed. Completed payments match order total = order is paid.

The case you are talking about is when customer pays using different payment methods for different items. It will be relevant when we introduce shipping from multiple locations - customer will pay per shipment or something like that.