Is it normal for a couple to have their own blankets?

Yes.  They can use them without the other "stealing" the blanket if they have 2.  
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it's what they are doing. Maybe one or the other pulls the covers off during the night, so this is their solution.

It's weird that you are concerned about their sleeping habits. Maybe they share a bed, because they can't afford twin beds, or there isn't enough room.

Eliminates fighting for the covers I guess.

Maby ones a blanket gog

Not everyone has the same bed time comfort levels. People have their own desired pilow, blanket warmth, room temperature, mattress firmness/softness and so forth.

Getting two people to desire the exact same sleeping habits does not magically happen between couples. It is normal for people to sleep in a way they felt comfortable.

Perhaps one of your parents is a sleeping blanket hog and the other one is tired of waking up in the middle of the night without a blanket, so they sleep with their own blanket. 

Sure, why not? Some people prefer this. 

good idea......................

My parents do the same thing too. My mom said it was a common Europian thing.

Kind of weird. I WANT TO BE UNDER THE SAME COVER. It isn't a deal breaker but I like to sleep engulfed by my husband's body. So what is the other blanket for? Lol