You need to specify in your event which tilemap needs to be engaged. Set your drag and drop behaviours to be disabled by default, and enable and disable the behaviour on-the-fly through events to ensure they only affect one object at a time. Your code should look something like this:
On touch object
| Pick all object
| Pick top
I've used 'Pick top' because it is predictable. When the user clicks on an area containing multiple objects, only the one that's visible in that area gets selected. You can use other criteria of course.