In respect of exchange of goods, namely gold watch for restaurant services will the transaction be taxable as two different supplies or will it be taxable only in the hands of the main supplier?

No. In the above case the transaction of supply of watch from consumer to the restaurant will not be an independent supply as the same is not in the course of business. It is a consideration for a supply made by the restaurant to him. The same will be a taxable supply by the restaurant.