Each product can sync through REX into different eCommerce sites. Once the product is enabled for export it will flow through to each sales channel using the source group that is selected for it. This can be changed for each product.
To view these options, log into your RetailExpress and search for the product and open its "Edit Product" screen.
Scroll down to the bottom of the screen and below the "Export to Web Service" box there will be an "ONLINE" and "WEB" are selected, as seen below. If you purchase another connector you can have unlimited second or third websites to also offer the product.
To have two Sales Channels active simultaneously, click on the Sales Channel on the Left-hand side, and make sure a source group is selected on the right hand side. Repeat this process for all Sales channels to verify that they are active.