I have a friend who owns several rental properties. He has had issues with renters who bring their own dryers. If the house has 3-wire dryer outlets, the renters need a four wire outlet. If the house has a 4-wire dryer outlet, the renters need a three wire outlet. He could make the renters change their cords, but some would do this themselves and perhaps do it wrong. He also had one renter change the outlet himself, and didn't realize it until after the renter moved -- again, a potential safety issue if it was done wrong. Some of his renters are elderly, or on some form of subsidised housing, and bring dryers from their old houses, or buy used appliances.
He ask me if it would be permissable to install one of each time of outlet at the dryer location, keeping the single feed from the breaker box. I have never seen this done, but can understand why he wants to do it. He does not want to run a new feed from the panel to the second dryer outlet, simply put one of each type at the end of the existing runs, which were upgraded to 3-wire with ground in all houses.
He ask me if it would be permissable to install one of each time of outlet at the dryer location, keeping the single feed from the breaker box. I have never seen this done, but can understand why he wants to do it. He does not want to run a new feed from the panel to the second dryer outlet, simply put one of each type at the end of the existing runs, which were upgraded to 3-wire with ground in all houses.