List most recent items combined from Channel and Structure sections
I was presuming that the default Order for Channels and Structure was postDate desc. But it turns out the default for Structure is sortOrder

{% set homepageEntries = craft.entries({
    section: ['channelOne','StructureOne',channelTwo','StructureTwo'],
    order: 'postDate desc',
    limit: 15
}) %}


Explicitly stating the order seems to fix this issue