I've been out of DDR for many years, but in an effort to get a little more in shape, the wife and I picked up two Champion Metal DDR pads off a guy on craigslist for $275, still in reasonably good shape. He used them with his old PS2, which I intended to use with Stepmania.
He had ordered them from DDRGame.com - and it came with this adapter for the PC, made by Mayflash:
Background: I have two machines - one is Windows 7 Ultimate 64 bit - the other is Windows 8 64-bit.
Try 1: http://www.ddrgame.com/ps2-accessory-dual-usb-adapter.html
On both machines, I started with one pad, but had the issue where pressing left and right arrows simultaneously would not work - luckily holding Up + Start + Select (on the controller) for 3 seconds each time it was plugged in worked, however, when plugging two controllers into the same adapter - it went absolutely nuts, rapidly firing buttons, the joy pad buttons didn't accurately map - lots of issues. I did have the latest driver from Mayflash.com.
Disappointed, try two was this adapter, the EMS USB2:
Try 2: http://www.amazon.com/gp/product/B008FG72X4/ref=oh_details_o03_s00_i00?ie=UTF8&psc=1
I read good things about this one, but dispapointly on a 64-bit system, it recognizes the adapter, but the pads never actually show up as game controllers. I could even go into the config application that the drivers came with, and see that it was detecting input, but Windows refused to recognize it as a game controller - partially because I think the drivers are unsigned - and its a huge pain in Windows 8 to force windows to load an unsigned driver.
Try 3: HDE adapter
It was cheap, so I tried it anyway - but even with trying the Up + Start + Select it did not allow me to press left and right arrows simultaneously, so I didn't even bother trying to get it to work with both pads.
Try 4: Super Joy Box 5 Pro: http://www.amazon.com/gp/product/B001DU25UO/ref=oh_details_o02_s00_i00?ie=UTF8&psc=1
I got very close with this one. Installed the drivers, hooked it up - and with this one you can go into the driver settings and set it to "DDR Mode" so you don't have to do Up + Start + Select everytime - I started getting excited. I mapped it to Stepmania just fine, but then when I plugged in two pads it started being really unpredictable. I could never predict which one it was going to make Joy1 and Joy2, and no matter what I did, at least one button would be "shared". For instance, I would step on the Enter button (top right) on Pad 1 and it would map to Joy 2 B8 in Stepmania. Then I would do the same on Pad 2, and it would also map to Joy 2 B8, so it like it was sharing the same joy stick buttons. I tried turning off auto mapping, tried remapping buttons in the button reconfiguration panel of the driver, and gave up.
Try 5: Combination - Some success!
I actually hooked up two different adapters. I used the adapter from Try 4 with only one pad hooked to it and also the adpater from Try 1 with only one pad hooked up to it. It still a little unpredictable as to which is going to be Joy1 and Joy2 - and I have to keep entering the Start + Select + Up on the adapter from Try 1 - but it's working now!
I'm curious if I could use two identical adapters from Try 4, but not sure I want to spend the money on it or not.
Hopefully this helps somebody, as I've been screwing with this for 3 weeks.