We read in the Talmud (Megillah 21b) that, like the Torah reading, two people may not chant the Haftorah out loud together. This is because it is very hard to hear each word properly, and the purpose of the public chanting is for the congregation to listen and learn.
As such, I would suggest that you discuss this with the rabbi of your synagogue. For example, it is quite common for one person to be called up for maftir and say the blessings on the haftara, while another does the actual reading. Whatever the solution, together with your rabbi, I trust that you will find a way to divide things up so that both boys will shine on their special day.
