You should also take into consideration that when you get there, there will be more expenses for food and activities (such as snorkeling, helicopter rides, submarine rides, etc).
I think you should call some travel agencies or look on the internet yourself. Could be the time of year (June after schools are out would be high season). One item that might save you a few bucks, as an example, is if your airline allows, you should fly into one island and fly out another (fly into Oahu, fly out of Maui, for example). That way, you would only be paying for 1 leg of inter island flight. Secondly, consider playing with the days/time if you have such flexibility. Your hu*****and's meeting days are set. So you can consider visit Maui first and then go to Oahu for the meeting. You might avoid a peak time that way. Thirdly, did you buy this as a package? A smart agent should be able to combine for you rental cars on 2 islands to save some $$$ there. Also, many island hotels offer a free night after a certain number of nights with the same chain. Some it might save you some $$$ to stay with the same hotel chain on both islands.
In Maui, it costs you a lot more to stay at Kanapali. But Lahaina, a short drive away from Kanapali, is a lot cheaper.
Just some ideas for you to play with. By the way, I personally like Big Island better than Maui, which is too commercialized.