So I keep learning about other similar products.
 
In addition to the 2 Technical Pro Products described here, I have discovered a number of others.
 
What I will describe here are all similar in at least the basic form factor.  They are rack mountable Dual MP3 players/mixers.  As far as I know of this writing, they all fit in a 3U rack space, but are also designed to be operated just laying flat on a table.  They are likely intended as a "drop-in replacement" for the control head of older Dual CD Players, but of course not needing the separate (also rack mount) Dual CD Player (Drives).
 
It's not my intention to single out a particular product, but I am going to use the PYLE-PRO PDJ480UM as most typical of these products.  It is pictured here:
 

                                                              PYLE-PRO PDJ480UM  Photo from Pyle Website
 
PYLE-PRO PDJ480UM
The same product (although unbranded) seems to be available from Chinese manufactures/vendors listed simply as "DJ Controller, Supports USB and SD Card Player".  I have also seen what appears to be the same product branded as the "UC3".
 
Numark DDS
This one appears slightly more elegant than the other products mentioned here.  It has a single Color display in the center.
 
Pyle-Pro PDJ450U Pro DJ Media Player Controller
Another Pyle branded product, and like the Numark DDS, it has a single screen in the center.
The same product seems to be sold as the T&W International Audio Co.,Ltd SDJ-1000US  as well as possibly other identities via  various Chinese Vendors.
 
The next several products seem to be the same device with different cosmetics and branding:
 
American Audio SDJ2  
JB USB 900  (JBSystems)
Citronic MPSD-2
OMNITRONIC MMP-1 SD/USB
UD999 (Unbranded)
 
The UD966 (Also Unbranded) seems similar.
 
Another one is the American Audio SDJ1 / SKP Pro Audio USD-2010 /  DJ Tech S1.  The same device may also appear with additional names/branding.
Note that these devices are just dual players with separate outputs.  They do not have mixers, faders, tone controls or Mic inputs.  I am leaving them on this list because they are similar devices.
 
Note that this list only contains self-contained units that do not include or attach to CD players.  Many Dual CD players/controllers will play MP3 files burned on to a CD.  This list would be much longer if I added those.  (Note reference to my Numark Mix Deck Express referenced earlier in this thread.  I also have a Gemini Dual CD player with remote head.)  
 
It is very possible that all of the devices listed on this page are obsolete, although some seem to be available.
 
Note that none of these devices seem to have nearly the sophistication of the Go DJ / Go DJ Plus, such as the spinning record animation or waveform displays.
 
None of these devices are portable/battery powered.
 
Note also that newer, high end devices, such as the Denon DJ Prime Go are not on this list.  I consider them to be in "Their own class" (and I'm sure Denon would not want the DJ Prime Go on a list with $150 obsolete products). The Denon DJ Prime Go was mentioned early in the thread.
 
If you have any additional information about the devices mentioned here, or other similar Dual USB/SD players, please email "John" at this domain, and request registration.