Here is a second specimen found directly opposite the first one on the other side of the stump. They were about 3 feet off the ground. Purple markings are different, but size is very similar. Is one a male and the other a female?

The life cycle for Eacles Imperialis says they emerge at dawn and mate after midnight the next day so perhaps both of these emerged this morning and climbed up this same stump to expand their wings. I wonder whether that means they are also likely to be siblings? Do they emerge a set number of days after pupating or is there some environmental trigger that got them to both come out the same day even though they started out underground at different days? Seems more likely they were offspring from the same parent who grew up on the nearby white pines and had a synchronous life cycle down to the day of emergence as beautiful giant adults. I should have looked around for others at the same time.