It's possible to add secondary Airtel numbers to Airtel Thanks app.
First click on the hamburger (three horizontal lines) menu. Go to services. Scroll down a little. Under 'shortcuts' card, you will see an option 'add existing connection' - that's it.
My UI was totally different. It seems there are total 3 UI of Airtel app. On my phone there was no option to add. Then I checked my father's phone. His UI was completely different from mine. But his one also didn't have the option. Then I went to settings and selected "Enable Beta". Only then I got the option under "Prepaid".
First click on the hamburger (three horizontal lines) menu. Go to services. Scroll down a little. Under 'shortcuts' card, you will see an option 'add existing connection' - that's it.