I've done this with a single 30' extension ladder and a 22' "Gorilla ladder". One advantage of the multi-position gorilla ladder is that the legs flair out a bit at the bottom for a bit of extra stability. The flair at the top allows the other extension ladder to rest fully inside of it.
The 30' ladder was extended only about 8 feet in order to reach my 20' ceiling. The other ladder was extended to a height of about 16 feet because extending it more made the whole thing hard to set up. So the 30' ladder actually stuck out over the gorilla ladder.
I used a pair of straps to hold the bottoms of the ladders together about 4' up from the ground. This stops the legs from spreading, and the straps are adjustable to fine-tune the height. A pair of straps is nice because they help stop the ladders from rotating.
(click to embiggen)
Obviously, you have to lash the tops of the ladders together securely, but you also have to tie the extensions to stop them from extending. The extension ladder mechanism is made to keep the ladder from collapsing, but in this A-frame arrangement, when you climb one ladder and lean back, it will cause the opposite ladder to extend. Each ladder must be tied so that it can't extend or contract before climbing. This can be a final step if you need to extend or contract the ladder to get it into place.
If using two of the same size extension ladders (not a gorilla ladder), the tops will not nest. They will be offset, and this will make it less stable than my arrangement. The top tie must be very secure to prevent rotation. Rotation will cause the ladders to tip and vice versa.
If you are in an open area, some type of outriggers or support on the legs to prevent tipping should be used. I was in a narrow area, so if the ladder tipped and fell it would have hit a wall rather than falling all the way.
Tipping side to side is the most important concern. I climbed to the top of this, and it felt rock steady the whole way, but the width is narrow compared to an actual A-frame ladder.