It depends upon the dimensions of your tilemaps - if one of the maps is larger than the other then the smaller one may be picked as being closer, even if the player overlaps more of the larger, because the origin of the smaller tilemap is nearer to the player's origin. There is also another issue: the tilemap object's origin is in the top left corner and can't be changed, so in order to find the centre you would need to add 0.5*tilemap.width to tilemap.X and 0.5*tilemap.height to tilemap.Y.
Could you explain what the new problems are? It will be easier to suggest solutions if you can outline all of the constraints.