Bruce, the white decoder is 99% likely a Soundtraxx DSX as others have said. Soundtraxx has user manuals for their older DSD decoders upon which the DSX was based, and a DSX addendum on their site under "old models" or such like.
Fortunately CV29 is one of the NMRA DCC standards that most decoder manufacturers follow well. So you can use CV29 to switch between long and short addresses the exact same way in pretty much all DCC decoders.
There are several CV29 calculators online. Below an example from the Digitrax site. In my example, I first set the short address in the NCE to be 34. And I set the short address in the DSX to be 12. I set the long address to be 1234 in both decoders.
Now if I set CV29 in both decoders to a value of 34 they both now respond to the long address 1234:
If I then set CV29 in decoder(s) 1234 to 2, both decoders will now respond to their short addresses, so the NCE becomes 34, and the DSX becomes 12.
Now I can program other CVs in NCE 34 and DSX 12 separately as needed. And when I'm done, I set CV29 in both 34 and 12 back to a value of 34 so they both now respond to address 1234.
Note that CV29 being 34 vs any other value has some other effects that you can see in the calculator. The important bit for this exercise is that you choose between short and long addresses! 
BTW, if you can get JMRI DecoderPro, this can be a lot easier than using a throttle for programming.
Pete
