Specifically - are there any ATX PSU's with double 24 pins, or some kind of adapters that would make it possible?
I have never seen a device as you describe, and would guess it does not exist. I believe it would be difficult to produce it and still live up to (among other things) industry safety standards. I could be wrong.
However, it would be simple to create one yourself. Just get an extra 24-pin connector with as long wires as possible from somewhere and connect it in parallel to the one already on the PSU. You can use something like these to make the connections.
Just make a note not to connect the "power on" wire to more than one of the connectors, or you might get surprises. I won't attempt to explain which one it is in text, see Wikipedia in stead. They list it as pin 16.