Hi,
There is no technical reason (there may be payment gateway terms and conditions etc that prevent it) why two shopping carts couldn't feed in to the same gateway account.
However some things to consider might be in relation to any feedback mechanism from the gateway to return a shopper back to the website on a completed/cancelled purchase. Or to transmit paid for order details back to the sending websites's shopping cart for stock control etc. Therefore, it depends on how these return URLS can be configured with these payment gateways, such that the sending site can specify their specific return URL. Also things like if the payment gateway shows a business name and you need this to be different depending on the website it came from.
In our case for instance, you create the products once in your Shop Integrator admin console and can plugin in your online shop in to multiple websites using the same set of products. This is because your shopping basket is a central service so any payment gateway data feedback for stock control or to trigger digital downloads etc will come back to your central service shopping basket service rather than have to be directed back to the initiating website.