adding chicks

Unless you will be housing them in separate quarters, sorted by breed, you will not have a flock that can support two roosters - if the birds will all be housed together you will want to get pullet chicks only.
As for integrating the new birds, you will want to brood the new babies separately until they are a good size (12-14 weeks) - then start housing them so that both groups can see each other, but not have physical interaction (ie side-by-side enclosures, a temporary enclosure within your current one, etc) for a couple of weeks - then allow mingling. You want your new girls to be mature enough to handle the attentions of your rooster when the mingling is allowed.

 

Good advice here. Not only will your flock not support two roosters, you will most likely have fighting as the two males work things out to establish dominance - especially if you add another mature rooster. Once they get things settled, it does not mean they will stay that way. Occasionally, a subordinate rooster will take on the dominant one just to make sure things haven't changed. Sometimes rooster fights can be to the death.

Housing them in an look but don't touch arrangement can help, but don't be surprised if there are still squabbles to establish pecking order once they're all together.