I can't tell you how many times I've run several extension cords together, out to an electric tool or the heater in the horse's water tank only to discover that I had no power. 99% of the time it was because my cords came unplugged (the other 1% was that one of my cords was actually broken or that too many things were running off of one breaker and it'd end up tripping. Grrr!). Annoying to say the least until someone showed me an ingenious little trick to keep the cords plugged together, no matter how hard I tug on them.
Cross the two ends of the cords over one another . . .