Consider there are three products in your store A, B and C. C gets a discount only if customer buys 2 from product A and 2 from Product B.
Step-1 : General tab
Give the basic details of your discount rule and choose Dependent product based discount as your discount method.
Step-2 : Condition tab
The following each product should have quantity 2 to get discount 30% for Product C = Mugs.
Product A = Clock
Product B = Mirror
So the condition should be like
Buy = Each - Equal - 2
Product(s) from = Clock, Mirror
Get discount in = Mugs
Step-3 : Discount tab
Choose discount type - Percent and set the discount value - 30
Save the rule.
Step-4 : Frontend demo