shop at xyz.com and secure site at abc.com - will it work?

I have a few sites, but only one is secure https

I want to be able to have things for sale at https://www.xyz.com

When people click on a product, it gets added to the cart at xyz.com, then they keep shopping.

When people click checkout, I want it to go to the secure site, https://abc.com to get the details and do the payment etc.

I have figured out how to make the products at xyz.com go to the cart, and figured out how to make xyz.com look at https://abc.com for the checkout system.

I have copied all the files required for cart onto both xyz and abc.

I need to configure another cart on https://abc.com so that it will process the payments - but what little tricks are involved with this?

What differences will there be in installation for https://abc?

At the moment, it treats things as two different carts.

Most importantly, where should the database be? At xyz.com or https://abc.com?

Many thanks from a confused Aussie! :)