Those are the old rules. There are new ones now. You can introduce your new two chicks immediately as long as you have a secure run and provide a safe chick pen and a little heat for them to warm themselves as needed.
This early exposure before the chicks grow to a size where the older chickens see them as competitive makes for a much easier and smoother integration.
Let the chicks spend a few hours a day when the weather is nice out with the adults. Increase the time as the chicks grow more cold resistant. By six weeks, the chicks can move into the coop with the adults.
Requirements are a secure run and a pen within the secure run so the chicks are safe, then around two to three weeks, you may open little chick size openings in the safe pen so the chicks can mingle with the adults but be able to run back to safety as they feel the need. Food and water are to be provided inside this safe enclosure so they don't go hungry.
If you have no run, well, I can't help you. It's just going to make everything much harder on the new chicks. Typically, new chicks will not venture out from cover until they're close to eight weeks or more, and if they do, can be easily picked off by predators. If you have no run, I suggest you get busy and build one.