I find that a paint can opening tool works well as a marlinspike for untying two ropes knotted together with a double fisherman's knot. It has a handy handle and a smooth flat hook on the end that works well to pry the knot apart.
An old utility climber told me that they used to tie rigging knots that were to be heavily loaded with a dry stick in the middle. He said they would break the stick in the knot and the knot would loosen up.
I look for a stick that is about twice the diameter of the rope. Finding one with a taper is a bonus. Most of the time I shave off the bark if its green. If not, knock off to flakes.
A wooden mallet works well to beat knots apart. Marlinspikes and screwdrivers too. Clamping the knot into a wooden jaw vice keeps it from jumping around.
In the end, sometimes the only cure is the ax though! Cut it off and seize the new end