I dont think heat is the answer, you would be expanding the aluminum in the fork and that is anti progress.

I have mounted a 2x4 in the vise and the fork went over it, then I could work the bars back and forth just every so slightly to try to break the bond.
also could be good to throw a bare hub in front to keep the legs from bowing out

I did this with the stuck seatpost in the tandem, slowly,eventually it moved 1mm,
after that its easy,
it was slow left right flexing and not too much torque, I really worried about it breaking.