Here’s what I did (I used your stretch.blend file), step-by-step since I have a lot of time:
First, make sure the vertex groups are set up the way you want them. Then animate the bones so they’ll move outwards from frames 1 to 6: with the Armature selected, go into Pose Mode. Select both bones, insert a Location keyframe, go up to frame 6, select one of them and move it outwards, select the other one and move it outwards, select both, and then insert another Location keyframe.
Go into the Logic Buttons. Select the Armature and the Plane. Delete the Collision Sensor on the Armature, but set one up on the Plane. Connect it to the AND Controller on the Armature. Change the Armature’s Actuator to Action. Set the AC: field to “Action”, set Sta: to 1, and set End: to 6. It’s probably also a good idea to set it to Flipper too so that when the object passes through, the hole will close. Now select the Cube and set the negative dLoc Z Actuator (act) to something much more extreme, around -0.1. Do the same with the other one (act1), except around 0.1.
Now when you press P and press the Uparrow, the Cube object should move down, and the hole will open up when it touches the Plane. If it doesn’t, you probably did something wrong so go back and check it. If it’s still not working, I probably wrote something badly so just ask and I’ll try to be more clear.
To summarize, you need to give the Plane a Collision Sensor, and give the Armature an Action for opening and set it so that when the Cube touches the the Plane, the Armature’s Action will play.