are there any others in addition to the two listed?
Every carpark is a dogging site, and MK is full of carparks.
However, the MKparks trust is very on-the-ball about what goes on. As a consequence, practically all have either been seriously cut down, or closed.
If you manage to find a decent one they have not closed, and you have some fun, DO NOT TELL ANYONE.
PM with some decent ones to follow