Agree with PPs. Diamond Head is best done early in the morning - both because parking can be a challenge, and also because the trail can get very hot and very crowded the later in the day it gets. We were also sweaty messes in our athletic clothes and sneakers, and would not have wanted to do anything afterward before we showered.
With Pearl Harbor, if you are not able to secure a tour time 60 days in advance online, you really need to get there early to wait in the standby line. And you want to be towards the front of the line so you don't have hours to kill before your tour time.
Because of the logistical issues, we did them on separate days - both in the morning. We were done with both by 11am.