I need to set up a second air conditioner in a room but I already have a 20k BTU portable unit on the same circuit in a different room and I can't run them together so I was thinking of getting a 6 foot extension cord for the smaller unit. It would be on most on the time for about 2 or 3 months.

So far my options are a 10/3 SJTW from Honderson (which lights up so I'd rather avoid this one if possible since it's in a bedroom), a 12/3 SJTW from either EP or Greatide and a 14/3 SPT-3 from EP. Based on what I've heard I should be fine with either of the 12 AWG cords but I just want to make sure.

Are there any other precautions I should take setting something like this up other than frequently checking the wires, connections and the air conditioner itself to make sure nothing is heating up? Thanks in advance!