Great question. As a general rule, you do not license to retailers, but to distributors/manufacturers. Retailers buy products from the distributor/manufacturer and sell it in the store.
As for multi-license deals, yes it can be done. Most licenses are exclusive though. The benefit to the licensee is that they gain a competitive advantage by licensing the product from you in return for a royalty. To make a profitable return on those payments to you, exclusivity is almost a must.
If you make significant changes, knocking yourself off so to speak, you will most likely need to file a different patent and then license it to the 2nd company. I saw this done by Romie Revson, inventor of the Scrunchie. She had a design patent, licensed it to Scunci and then changed it, filed a new patent and licensed that patent to Goody Product.
Hope that helps.