Well, when you pay for an order on an ecommerce website, you're redirected to the payment gateway of the payment method you selected at the end of the checkout, you make the payment for the whole order and then you're redirected back to the website.
If you were to select different payment methods for different groups of products in the order, it could mean that half of the order would have to be paid for example on PayPal and the other half on Authorize.net. But that's not something you want as it is not user friendly. So we didn't implement that possibility.
That's why I'm saying that you can't have that.
That's the way all ecommerce solutions and ecommerce websites I know about work so I didn't feel necessary to write that anywhere. In fact, you're the first person trying to do that in 5 years with Hikashop as far as I know.