What to do when 2 or more PC’s fight the same monster

I am very new to D&D. I am making my own campaign to start to get my family, including my 8 yr old son interested in it. I have much of the basics down, but have one question.

Is it possible for 2 or more players fight a monster? If so does that change the dice count? Here is an example:

I have a monster that I have PC’a us d20 on, 11-15 hurts it & 16-20 kill it. But, if 2 players gang up on the monster, do I do a 40 point scale, or keep with the 20? I don’t want to make it too easy, but some how increasing the monsters life based on the # of players doesn’t seem fair either.