Roosters will be roosters so yes, there will be drama at the beginning. They are going to fight. If the rooster you are adding to the flock is a youngster, once his hormones kick in he will try to push the limits with your dominant rooster and try to depose him.
I have a LOT of roosters so there isn't anything they do that surprises me any longer.
The youngster may depose the older bird as flock master or he will get his attitude adjusted and fall into the roll of 'First Lieutenant' and help the dominant rooster watch over the hens. The dominant rooster may let him mate the occasional hen but if he tries to mate a hen that the dominant rooster looks upon as a favorite, he will chastise the second in command.
My flock leader is a one year old Egyptian Fayoumi named Tut. His second in command is a little BBR rooster named Stoopid. I currently have one hen brooding 4 7 week old chicks and a second hen who just went back to the flock, leaving her three bantam chicks for the roosters to watch over and yes, they do guard the chicks.
So having a second rooster is a good idea, once you get past the dominance silliness they should settle down and get along.
Also, if the second rooster is younger, they may just slide into an 'understanding' with little drama.
You don't know which way it is going to go tho so I agree, no matter what, be diligent and ready to step in if things get out of control.