You can add the BuyNow module to multiple pages in your DNN portal which allows you to create of an unlimited number of stores on your Web site to offer different product selections, presentations, payment methods, shipping rates, and tax setup, etc.
If the share checkbox is checked for a particular product the product will be displayed in all product catalogs on all pages where you have added the buynow module. If the share checkbox is not checked then products added to your first cart module will remain separate from products added to your second cart module.
The share flag does not give you the ability to choose which buynow module to display a product in. In order to implement what you are looking to do there would have to be a list box in the product setup screen where you can select from a list of buynow pages that you want the product to be displayed in. Would be a nice feature to have in the cart but its not implemented yet.
A possible solution for you is to create a master spreadsheet of your products once and then import it to each of the buynow modules that you are sharing products. Then delete the products you dont want from the individual buynow modules.