+1 for combining the penetrating oil, time, and Jedduh01's bashing method.  It was described to me as going berserk and attacking a tree with the fused cone/trumpet acting like a war hammer.  

Firmly grasp Metal trumpet in hand = Cone pointing away from you

Raise assembly high above head.

Briskly + Forcefully  swing and smash  Part firmly onto ground  block or such to create a forceful impact.

 Rotate slightly

Repeat:

They will sepearate.

You'll then need a long drift or Punch rod thru the trumpet to drive out the Knuckle Joint


Or = like mentioned... Shelf the whole assembly  = Install all new Knuckles + Hi Lo's and Cones. in one go.
  spent more time than should have for this job =  1 new cone on LH lineup vs the 4 removed from the car.
Restored the ride COMPLETLEY!

try soaking the whole thing in a bucket of white vinegar. I recently learned and use vinegar as my rust remover. Works great. I use a 5 gallong plastic bucket and when I dump a gallon or two of vinegar in there, I'll fill the empty jug with water and use it to raise the height of the vinegar. put a bag over the top to slow evaporation.  Works great. 2 or 3 days maybe longer, wash it in water, if it's steel, wipe it with oil to prevent oxidation until you paint it.

If you're putting Springs or high lows in just throw the whole things away as you probably need to replace the cones.

The rubber is intentionally bonded to the steel. The steel hemisphere and the aluminum trumpet bond because they are dissimilar metals.

first, get good penetrating oil in there.

leave it a while

heat it

get more good penetrating oil in there.

repeat often.

a large vice on a solid work bench can grab the rubber while leaving the trumpet free to move. You can put a snipe on the trumpet and move it one way or the other and it might break free or it might rip the steel hemisphere from the rubber.

you may also be able to get a sharp chisel between the rubber and the flange of the trumpet and get some good blows in there that may hasten the release.

Accept that things may break.

Sometimes they come apart easily. Other times the rust bonds the rubber permanently to the metal and the only solution is to carve away either the rubber or the metal, depending on which part you want to preserve. Sometimes the rear trumpets can be separated by swinging them against a hard surface so only the cone hits.

By the way, your question might be clearer to other if you used the common names for the parts. The names will probably make more sense once you see them apart. The rubber part is called a "cone" - it is a rubber spring that, in its original shape is roughly conical both on its outside and its inside.  The metal part is called called a "trumpet", a little more apparent when you look at the rear trumpets.
In the attached photo, there are 3 rubber spring "cones". The one in the middle is the stock shape of a new cone. The one on the left is an old, worn out and deformed stock cone. Yours probably look like that. The one on the right is a "Smooth-a-Ride" cone, a later design to provide a much softer, compliant ride when not heavily loaded.

Looking for tips how to separating the rubber disk from the cone suspension.