"The area under the bed is darker than the area above the bed."
Translation:QongDaq bIng Hurgh law' QongDaq Dung Hurgh puS.
QongDaq bIng means the area under the bed, and QongDaq Dung means "the area over the bed. (Remember that the word QongDaq bed* is not Qong plus the suffix -Daq.)
QongDaq bIngDaq means to/at/on/in/by the area under the bed (I'll simplify that to at the area under the bed, but it means all of those), and QongDaq DungDaq means (simplified) at the area above the bed.
You're trying to talk about how the area under the bed is dark, not how the at-the-area-under-the-bed is dark. That's why you don't use a locative suffix there.
It's unclear that we can use locative suffixes as the noun arguments in comparative sentences anyway.