ecommerce website with inventory tool

We need to make two websites that has identical "backend" structure with different front end designs. Design will be provided, but if design can also be done, we're open to it. Design must be high-end. The website will feature a shopping cart / ecommerce function. This is an aftermarket auto parts manufacturer (company 1), and retailer (company 2) website. Users need to log in and "add to cart" the products. The admin has to be able to easily upload and maintain the inventory (catalog) using a "site management system". We do have access to older "scripts" that has similar functions, but we're more willing to accept "fresh" scripts from scratch. We're also open to utilizing open-source codes if it makes the job makes it easier and faster. We will pay in CDN dollars, so please bid in CDN dollars. This is an urgent project. Needs to be completed in less than 3 weeks.