Marker display preference

I have a map that has two different colored markers. One represents
perferred locations and the other represents non-preferred locations.
In the list coming back from my database, the preferred ones are first
(mainly because I list them down the side next to the map). What I
have noticed is that when doing a city search, sometimes the preferred
maker is behind a non-preferred marker and sometimes it's vice versus.
Is there any way to display a marker without having another marker
overshadow it? I realize from a proximity standpoint that one has to
be displayed before another one, but I was wondering if there is a way
to determine which one gets put on first. I thought it was based on
the ordering if the load of markers, but it appears as it's more of
starting at the top of the map (north) and going down, so that which
everyone is furtherest south gets displayed on top of the other one.