You should order Mathias' and Elizabeth's death certificates. Those should have their burial information on them. See:http://www.dhs.wisconsin.gov/vitalrecords/index.htm
When people are buried in the same grave, they usually aren't buried in the same coffin. Relatives usually buy a large plot of land and "add" to it.
No, I'm not related. I just like to try to find genealogy info! :)