Add item to cart automatically when another product is present in magento cart
<p>I want to add item(one fix) to cart automatically when another product is present but quantity changes when main product quantity changed. Ex. if I add Product A with qty 1 in cart then product B with qty 1 will be add to cart same if I add Product A with qty 2 in cart then product B with qty 2. Please don't recommend any extension. Thanks </p>