Separating a "stuck" carbon mast ferrule can be one of the most frustrating things that can happen in windsurfing.
Here's what I have used on more than a dozen "two piece turned to one piece" masts than others have given up on.
1/ Find 2 elevated sufaces (saw horses...car bumpers... tall wood blocks) that will allow you to bend the mast at least 12" (30.5 cm) in the middle.
Put some padding on the elevated surfaces to protect the mast.
2/ Place the elevated surfaces far enough apart that you can bend the mast without the ends slipping off your fulcrum points.
3/ Push down and release the mast (bend it down below the centerline between your elevated surfaces) rythmically many times, and rotate the mast every few cycles.
Listen carefully to the sounds that bending the mast makes.
If you hear "crunching" from the sand caught between the inner and outer ferrules, work more in that area (at that particular angle).
The "crunching" sound is the trapped sand breaking down, and this is what you want.
Break the sand down only in the area where it's trapped, without damaging the other areas of the ferrule by twisting, trying to "wrench" the ferrule apart by "spreading" the sand granules around to other areas of the ferrule by twisting them with booms.
Twisting with 2 or more booms also runs the risk you will "crush" the mast as most masts are not reinforced for "crush" in the areas adjacent to the ferrule.
4/ After working the mast up and down (benidng it) repetitively until you hear the "crunching" sounds go away, try to pull the two halves apart. If you twist the mast
slightly as you pull (only twist it a few degrees, NOT all the way around, and then twist it back and a little beyond the angle at which it was stuck) it may pull apart easily as the sand/grit has broken down and it will not keep the mast stuck together.
If the above does not work, here's a bit more expensive (for the hose and fittings) method that has worked to separate masts that the above did not sucessfully get apart.
Since you can take the top cap off, you can get a short section of rubber or plastic hose
and a couple of plastic reducer fittings, hose clamps, and a plastic cap.
Get a section of hose that fits the top of the mast closely and then use reducers to get a garden hose fitting on the other end.
Use a couple of hose clamps to seal the hose onto the mast the hose onto the hose fitting.
You may need to use a larger hose cap, 2 hose clamps, and a larger diameter section of hose on the base of the mast to seal that end.
This method will push any plugs in the lower section of the mast out the bottom, but it normally does not damage the plugs and you can stick them back in the mast.
Once you have both ends plugged, connect a garden hose and set the mast up on elevated surfaces like the above dry method.
Turn on the water to the garden hose and let the water "flush" the sand from the ferrule joint.
Work the mast just like the dry method, and listen for the crunching sound.
After a while, the water will begin to seep out of the joint, and once it begins to flow a
bit, it will flush out the grit and your mast will come apart easily.
Tips to keep this from happening again:
Use soap (regular bath bar soap) on the outside of the male part of the ferrule.
This will prevent sand from getting into the joint, and lubricate the joint so it will come apart more easily.
Get a roll of vinyl electrical tape, and put about a 1 1/2 wrap around the joint line between the upper and lower sections of the mast.
This prevents all sand and water from getting into the joint, and helps to ensure that your mast ferrule stays full together when you are rigging your sail.
This does not work if you are using cambered sails as the cams will hang up on the tape
Avoid rigging you mast/sail on the sand, but if you must, put the mast base in the bottom of the mast before you get to the sand, and be careful to keep the joint above the sand.
If you drop the ferrule (either side) into the sand, take it to the water and throughly rinse all the sand out of it before putting the ferule together.
Hope this helps,