This is going to depend on your set up, and your space. Measure the set up, show us pictures of the space. IF you have a farm set up, then maybe this would work, if you have a truly backyard set up, the odds of this getting ugly are very high.
How old are the grandchildren, if they are under 6, I recommend no roosters. Roosters are a crap shoot, and some go from the darling to the nightmare in an instant. They tend to attack children first, which if the child is under 6, tends to be at the face level. Often times people who have not seen an aggressive rooster, vastly underestimate how violent it can be.
If you have neighbors, the crowing can be very loud and irritating. And with two, they tend to have crowing contests.
Flockmates roosters, birds that are raised together also tend to become bullies, they outgrow the pullets, become interested in mating long before the pullets are ready, and because they are bigger, aggressively demand it. Your pullets will be rode hard, and ragged.
So yes, in my mind you are crazy to be thinking of keeping two. Personally, I would only add a second rooster if my flock got over 25 hens. I have been keeping chickens for more than a decade.
Mrs K