It's been a long time since I've worked in eCommerce but at my last ecom role we used two payment processors and simply stored the card data with both for every customer, and the relevant IDs in the customer record in the database. Whenever anything got charged we'd check which processor we were supposed to use and off we went.

Yes this has an increased cost if your processor charges by number of customers, but I don't think that's particularly common - these two were just revenue + monthly fee.