I have recently set up a magento multi store, for arguments sake we will call it site2.com, I also have a site1.com. I realised that no products are showing on site2.com (As I want both stores to share the same categories and products) So I investigated further and realised you have to amend each product to be visible on each website. Is there a way of mass doing this change?
There is inbuilt feature provided in Magento admin panel.
Admin >> Catalog >> Manage Products
Select multiple products checkbox and select Action dropdown as "Update Attributes" and click on Submit button. It will reload the page with new options. Just click on "Website" tab and make changes in "Add Product To Websites" section