I recently heard that the DX Doubler (DXD) SO2R Controller by TopTen Devices, has been discontinued. Looking at their Web Site, it has completely vanished, along with the Radio-specific cables, User Manuals, etc. This controller was a great addition to my shack a few years ago. It is a very simple design, with no Microprocessors involved, and is not only useful for SO2R use, but also a fine centerpiece in a two-radio shack for everyday operation. Given its Parallel Port interface, it is understandable that the DXD box could not be expected to compete with more modern USB-based SO2R controllers. Unfortunately, these newer designs appear to be much more complex and fairly expensive. I will continue to use the DXD boxes in my station for now, but will start looking at an eventual replacement.
As you may know, I have designed a USB interface for the DXD and will wrap-up that activity in the near future (update – completed long ago). I plan to go ahead with the final version of the PC Board layout for the DXDUSB, as the project has been a great learning tool, and I would hate to leave the project unfinished. The resulting board is really a general-purpose Arduino-based decoder for the Open Two Radio Switching Protocol that is supported by SO2R controllers. I hope to be able to use it in some other Contest-station supporting roles in the future. It may be useful as a Band decoder for some radios, or for interfacing of older radios with modern Contesting software such as N1MM+.
It does seem unfortunate that all signs of support for the DXD seem to have disappeared, considering that it was still being sold just a couple months ago, but I will be looking forward to future product offerings from the folks at TopTen.
August 1, 2018 Update
I continue to use the DX Doubler in my station, as do many others. Even after it was discontinued, I rarely see one for sale. The only concern with using this now-discontinued box is that the rig-specific cables are no longer available commercially. Since the interfaces are all audio connections, it is not very difficult to make your own. I have mounted one of my DXDUSB interfaces inside of my DX Doubler, so I now have what is probably the only DX Doubler anywhere that has a “native” built-in USB interface. It works great for me.
The DXDUSB has been completed and I now have several people also using it to allow them to interface to their DXD using only a USB port, including a few International users. This has been an interesting project. A new version of the PC Board has now been produced, that can be used for Band Data decoding, to allow using N1MM+ software to output the data needed to drive peripherals like a 2X6 antenna switch. This is still in the initial debug stage. It will allow me to provide customized or K3-compatible Band Data signals for switching 2 radios to any 1-of-6 antennas.
There are two well-know alternative commercial SO2R boxes that you can buy; the Microham and the YCCC SO2R controller. I have no experience with either of these. What I hear is that the Microham line is very flexible and also very complicated, using many Ports for computer connection. The YCCC box looks a bit more like the DX Doubler to me. I have considered getting one of these, but for now I will be sticking with my DX Doubler.
Kg3v Ham Radio Blog 
When the DXDUSB is ready or if you need a another field tester please let me know. It is disappointing that Top Ten stopped doing it to the extent they have removed it and cables etc from their site. I was thinking of doing my own or designing one and sending it to MFJ or DXE to build and sell. (I am not sure I have time to build and sell products. Maybe one or the other but not both.) I like what I have read on your site about the DXDUSB and your approach. My biggest issue was having both Icom and Yaesu radios and the need for different Mic types. Obviously for SSB I want one headset with the L and R radios in the proper ears. I was thinking of a small .1mfd cap in the phantom power line with a switch for I/Y on the line going to the radio. Of course there are times in could be 2 Icom’s or 2 Yaesu’s or Kenwood’s.
Tom, please give me your callsign. Would like to talk to you more about your thoughts on SO2R controllers. I considered building one too, but I just have too many other projects going on already. I use a single MIC with my DX Doubler and the IC-7600/FTDX-3000 radio combo, and it works great. I still need to finish my final DXDUSB PC board so it will fit nicely in a chassis. I would send you a prototype (may not be pretty) if you want to try it out. You would need to buy the Cisco USB–> Serial converter cable. It would be a couple weeks until I could send it to you. If you are interested, please email me at -mycallsign-@chesbayva.com
73, Tom