Can I use two ladders to create a makeshift scaffold?
Instead of a horizontal ladder, I'd use scaffold extension plank.  These are designed to work with an extension ladder or with ladder jacks.  Some have clips for ladder rung attachment, I've used a pair of c-clamps (just on the extension plank, to prevent it sliding)
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OSHA spec on plankings, contains span ratings for wooden planks 

Little Giant 8 -13 ft telescoping plank