Can you recharge a mostly dead 9 volt battery by plugging a fresh one into the terminals?

you’ll end up ruining both and possibly start a fire.

Batteries work using chemical reactions, and regular batteries cannot be recharged in this way.
And yes, you can start a fire this way.

DANGER! DANGER! Will Robinson!

You can not recharge batteries that are not designed to be recharged.

You actually can recharge plain old alkaline batteries but you need a charger specifically designed to do so. Most people don’t realize this because it is a fairly recent development.

But you can connect them in series for some healthy home made fun.

Don’t actually do this.

https://www.youtube.com/watch?v=8hwLHdBTQ7s

Regular batteries cannot be recharged.

@kritiper they actually can, with a special charger and if they have not been discharged all the way down.

I wouldn’t recharge regular alkaline batteries. The chemistry god gets very angry if something goes wrong.