Most of the Hiltons in and around Chicago charge extra for the second person.
A few examples: Hilton Northbrook $79/1 $99/2 best available (wknd)
Hilton Oaklawn $138/1 $148/2 best available
Hilton ORD $119/1 $144/2 best available
I started noticing this when checking Expedia to beat the Best Rates Guarantee. Would check, for example, Hilton Northbrook on Expedia and it would come back at $79 (while Hilton.com showed $99). When clicking "Book this room" on Expedia to print a screenshot of the total it would show the room @ $79 and +$20 for extra person.