As Centaurus stated
Depends on the position of the critical obstruction that affects each runway. Quite common to see different circling MDA for each runway
And as Tinstaafl has also stated,
an obstacle could intrude into one runway's missed approach but not into another’s
The MDA whether for a circling approach or a straight in approach is firstly dependant on the critical obstacles affecting either the approach or the missed approach gradients. The critical obstacles are often not the obstacles within the circling area for the particular performance category of aircraft. However once visual an aircraft on a circling approach is then subject to the rules applying to vertical obstacle clearance along the flight path and the visibility criteria determined by the procedure designer.
Hope this helps clarify the problem.
Blackburn