First of login to Magento Admin and Go to Promotions -> Shopping Cart Price Rules
Click on Add New Rule. (Button on top right corner)
Add a rule name and descriptions. Select customer groups.
Go to conditions tab.
Click on green + button and select option "Product Attribute Combination"
Now it should show you "If an item is FOUND in the cart with ALL of these conditions true:" Now click again on new + button at bottom of this text and select SKU from the drop down.
Step : 7
Now it should show you "SKU is ... " Click on three dots and Search the SKU of the product for which you want to set offer.
Now go to actions tab and select option "Percent of product price discount" from the Apply drop down.
Set 50 as value in Discount Amount Textbox.
Set two as value in Maximum Qty Discount is Applied To Textbox and save the rule.
That's it basically we gave 50% discount on total if there are product with 2 quantity in cart. This is how user has to pay for only one item and they get other item as free. Hope this helps you.