What did they do with the remains?
They were probably left in the ground to decompose further. This site has remained unoccupied by a new "tenant" for 5 years now.
fter 20 years the grave can be used again?
Yes, that`s what I wrote. If you don`t pay the grave fee every 20 years, your site is offered to a new lodger coz you don`t own the place, only lease it. :):) I saw such double graves in cemeteries.