Why not keep them both until they are fully mature. You don't really know what their personalities will be until they are 6 or 7 months old. If they start fighting one of them will have to go. But with 15 hens two roosters is not too many. I have two roosters now and as long as one of them is willing to be #2 they seem to get along okay. If they both want to be the#1 rooster then they may fight to the death.
I know a lot of people don't like roosters but I have had up to 7 of them at a time with no problems as long as there are enough hens to go around.
I know a lot of people don't like roosters but I have had up to 7 of them at a time with no problems as long as there are enough hens to go around.