How to set 2 objects colliding but not falling

Hello, can you help me please ?

I want to set these 2 objects

<img src="http://repstyleart.com/ask_froum_construct/piece_collsion.png" border="0" />

with these properties :

• can move by drag and drop
• can't fall (stay at the place it is)
• can't overlap the other object
• can't push the other object

I tried with the physics behavior but when i set the 2 objects to immovable they overlap. And if i set one immovable:yes and the other immovable:no, the immovable:no fall and the immovable:yes stay at his place and the 2 collide and don't overlap.

+

• can be placed by magntism and alignment (like on a chess game) according to the grey cases (not the white) of this grid :

<img src="http://repstyleart.com/ask_froum_construct/grid.png" border="0" />

Thanks a lot if you can help :)