Try in this order
1) If you can get the other bolt out you might find that you can rotate the whole cage taking the bolt with it.
this is the only option that will save your cage!
2) You'll have to sacrifice the cage to get mole grips (
http://en.wikipedia.org/wiki/Locking_pliers) onto the allen bolt.
If this doesn't do it then you are into damaging the bolt.
3)cut a slot into the head. Don't go all the way through the head straight away, the head of the bolt will provide some torsional strength.
If this doesn't do it then the bolt is seized, I'd use plus gas or some diesel and allow at least 24hrs to penetrate before trying again.
Trick - clamp your mole grips to the screw driver to provide the turning force while you use your body weight to hold the screwdriver in the slot - another pair of hands to hold the bike/screw/workpiece helps
4) as a last resort get a sharp drill bit, start with a very small bit. Try to drill as central as possible, use a punch to stop the bit wandering on start up. You may find that the heat from the drilling has loosened the screw and it will screw out straight away. If not go up a size of drill bit. Eventually you will reach a point where there is very little of the original bolt left.
If you are not sure what you are doing take it to someone who knows what they are doing!
This method of removing rusted up bolts has worked for me since I restored my first old classic motorbike in the 1980's