You could get some think plastic wedges and put them in between the "ears" of
the piston sleeve and the head between the two cylinders.  Then tap with a
hammer.  This is a real pain and I had it happen to me last year when working
on a customer's van.  I finally removed the sleeves along with the head.  I
then did a hone and re-ring job, and put the cylinders back on using a large
hose clamp (Home Depot) to compress the rings.  It was a bear and I probably
would have saved time and effort if I just removed the engine in the first
place (I didn't charge the customer for my extra time).
So learn from my mistake, if the cylinders come off with the head then just
pull the engine and save yourself alot of agravation (actually I think
pulling waterboxers is almost fun!)
Ken Wilford
Van-Again
John 3:16