Can the Si86xx have 2.5V on one side and 5V on the other side? Can I use it as a level shifter?
Yes, internally the Si86xx has two independent dies that are each powered separately. Either die can be powered from 2.5V to 5.5V.
Learn more about the Si86xx isolators: http://www.silabs.com/products/isolation/digital-isolators/si86xx-digital-isolators