Hi,
Authorize.NET provider supports different setting only per site, but you can create your own provider. Derive new provider from CMSAuthorizeNetProvider, override InitializePaymentRequest method and use credentials of your choice:
var error = base.InitializePaymentRequest(requestParams);
requestParams["x_login"] = "my custom login";
requestParams["x_tran_key"] = "my custom key";
return error;
You can access cart object through ShoppingCartInfoObj property.