I think I have found my answer.. It is possible with default magento itself.
So,
Step 1: I have created one group product(Made it enabled and in stock).
Step 2: Added associated product(All the product should be enabled and in stock)
Step 3: Now need to create the shopping cart price rule to give discount .
I have used sku to apply shopping cart price rule.
So I have created rule like this:
If ALL of these conditions are TRUE :
If an item is FOUND in the cart with ALL of these conditions true:
SKU is Test 12
If an item is FOUND in the cart with ALL of these conditions true:
SKU is Test 13
Step 4: Now need to give action, means need to set the discount price.
So the rule will be if both the product exist in the cart the discount will be applied.
Step 5: So the functionality is all set,Now need to do some modification about how you r going to display to the customer that there is an offer if purchase that two product combinedly.