Ok so there are two places you can get to here the first one is the chao location.
To get there Need to get to the second last checkpoint and then go back on the red rail. Now if you are metal Sonic making it up will be a brease just jump when you see a platform under the inverted tower. Now when you get to the chao there will be a life in 2 player mode. If your Sonic or Shadow you will see that the life icon stays the same but if your metal sonic or Amy they will have Shadow or Sonic life! Strange isn't it? Ok the next one is when your at the end of the level. So you know where that tunnel to the Gaol is right? Well instead of going into it go on top of it and keep walking until an invisble wall stops you next jump off and as soon as your falling under the tower do a homing attack and keep holding up and should end up at the place where the bits of the ark are following you down and if player two is near the bits will fall in front of you and you can push them without being hurt!

the second one seems cool, i might try it