I don't know how eWay works but usually, when you get money from your website on your payment gateway account, it is all summed up in your payment gateway balance.
Then, you can freely transfer the money to one or several bank accounts as you wish.
But you won't be able to know how much that should be for each product in eWay.
In that case, having several eway account would be a first step.
Then, you would have to duplicate the eway plugin (it require code change and should be done by a programmer).
And finally, you would have to restrict payment methods on manual shipping methods which would themselves be restricted by weight/volume to the corresponding products (no coding required for that). Or you could do it cleaner with the help of a programmer by having a custom field on your product to select the merchant account to use and then in the eway plugin decide which one to use based on the merchant account of the products in the order.