PowerStore is not configured to have two sets of checkout pages. You can either set the global file to use the PayPal gateway, or the Authorize.Net gateway.
If you have eCart, you can run the checkout wizard to create PayPal checkout pages. On the last step, make sure to give them unique names. On the cart page, you could set a Checkout with PayPal and Checkout with Authorize.net link.
The Checkout with PayPal would link to the new checkout page you created for paypal, the Checkout with Authorize.net link would link top the Powerstore checkout pages. I dont recommend this if you are using the inventory control features of power store since the new checkout pages will not have the ability to change the inventory on checkout.