They work, I've been using one for quite a while now.
I use the 24 pin so I can load the PSU then use a hand held LCD power supply tester to read the voltage values under load.
Couple of the PSU I have 1200/1350 watt have multiple CPU connectors and plenty of power for a dual board system, I've never tried it but theoretically it should work.
The 24 pin splitter I use sure saves a lot of headaches when testing PSU. Think I tested 5 or 6 last month.