Co-branding is usually understood as the use of two or more trademarks on a single product, each mark belonging to a different owner and pointing to a different source. A common example of co-branding is a brand A car with brand B tires and a brand C sound system. In this instance there is likely a trademark for the finished product and other distinct marks for parts or components of the finished product. However, when two trademarks appear on one product (i.e. a manufacturer’s trademark and a retailer’s trademark) the situation becomes more complex.
In a manufacturer-retailer situation, only the manufacturer’s mark can act as a source identifier of the product. For instance, when a clothing retailer places its trademark on price tags or labels affixed to products manufactured by others and bearing their mark, the source of the clothing is the manufacturer, and its mark should be indicative of it. In such circumstances, the retailer's trademark cannot act as a source identifier of the clothing which only has one source, the manufacturer. Perhaps such use of the retailer’s trademark instead is a form of promotion of the retail services it offers. Put another way, the manufacturer’s products do not become the retailer’s products simply because they were purchased at the retailer’s store and bear its mark.
It is not enough for a trademark to simply appear on a product to constitute use that generates trademark rights; the trademark must actually distinguish the products or services in association with which it is used by its owner from the products or services of other traders. In other words, the consumers must associate the trademark with a source of the products.
The only way the retailer’s mark put on a product manufactured by someone else could act as the source of the products would be if the manufacturer’s mark does not appear on the product, as is often the case when retailers enter into an agreement with manufacturers to make products that exclusively bear the retailer’s mark (also known as private label). Retailers only applying their mark to products already bearing the manufacturer’s mark do so at the risk of seeing their trademark application for that mark refused if it is successfully opposed by third parties, or their registration invalidated, if their registration covers those products.