I have used multiple / attached extension cords to run my pond equipment (heater / fountain, etc), but this summer I found two of my extensions "fried" together ~ oops!!! ~ and now no power flows through them ~ oops again!! I put the connection into an empty bleach bottle (cut a hole in the bottom that the plugs will fit through) and set it up on a block so rain (or snow) doesn't get it. So... it can be done. HOWEVER.... anybody I think with electrical knowhow will tell you that it's not safe! Your best bet is to run an underground cable (use proper outdoor electrical wire enclosed in a waterproof pvc pipe) from your electrical box to a GFI plug in your coop. I understand this is a very "basic" operation. All I'm saying is you (and I) are taking a risk when we use extension cords outdoors for any significant period of time.