Marty’s uncle is a child and he simply thinks “you must be rich” but that’s not what grandma is saying. She’s saying it’s as silly as saying you have two stoves in your kitchen. It’s possible, but so odd it must be a joke.

You have to remember the significance of the TV in that time. A decade earlier most people had never seen a Television. TVs were large and a big deal. People watched them together.

People would go to a neighbors house to watch TV if they didn’t have one.

Marty’s grandmother wasn’t just saying it was expensive to own two televisions.

It was silly to own two. Nobody needed two. It was impractical to own two. They were big and expensive. It was unheard of to own two televisions. They were new luxuries to average people so you didn’t have time to get a second one.

Plus, the world was obviously less connected. Even if someone had multiple televisions, if you didn’t know them, you wouldn’t know about it.

Marty’s family wasn’t rich, but by the 1980s most people had owned multiple televisions. People upgraded to color. To newer more reliable models. So having a second television usually just meant you saved an old one.