How to maintain one inventory for two stroes

[nachi19]

Hi I have two stores one is webstore and another is retail store i.e epos , i want to maintain the same inventory for these two stores. So i have changed in the store settings of myretailstore 9100 of Inventory facility id to web store warehouse, but the problem is when i make a sale for the product in the epos the inventory is not reduced for that product which i have received under the webStorewareHouse facility instead it creates a one more inventoryItemId automatically under MyRetailStore and it will be in a negative value.. can any one tell me how to maintain the single inventory for two stores.