* Agricole to hold majority of new entity (Adds fund manager comment and more detail): * Business line tie-ups easier than full-blown mergers (Adds detail on advisers, stock price and more detail) PARIS, April 17 (Reuters) - France’s Credit Agricole and Spain’s Santander plan to combine their custody and asset servicing operations, in a deal that could point the way for European banks to achieve scale without the complexity of a full merger. The new business will have around $3.8 trillion of assets under custody, closing the gap on European leaders and providing scope for savings and cost reductions.