To understand working of Cellular iPad+iPhone and number of sim cards required I am totally confused by iPad Cellular calling features.

If I put a sim card in iPad cellular, how do I make or receive calls in iPad since, there is no default dial app in iPad.

Case 1: 1 sim card: Doesn't work for me.

Suppose I have 1 sim card and I insert it in iPad Cellular. I can browse internet, but can't make phone call. And iPhone can't make calls either as it has no sim card.

I would have to download some Apps for iPad to make calls. Some apps don't work in some countries. So which app is good for this scenario?

Case 2:  2 sim cards: Works, but is this what iPad Cellular designed for?

1 sim card for iPhone and other sim card just for iPad for browsing internet. 1 phone number is wasted here. If I have to use Wi-Fi to browse internet by setting up iPhone hotspot, I would have bought a normal one instead of the Cellular version.  

I need to understand the working of Cellular iPad and number of sim cards required.
No. iPads just use SIM for data, not voice. 

You would need to keep your SIM in your iPhone and use the forward calls and text feature for iCloud to receive and make calls and messages from the iPad through the iPhone. Also, look into WiFi calling on the iPad by enabling it in the carrier / phone section of your settings app on iPhone.