I have a Bachmann EMD GP40  - DCC SOUND VALUE Equipped Locomotive purchased on Oct. 1st 2023 which is running fine.

I wanted to get another DCC locomotive to run consists with this one and purchased another GP40 but only DCC ON BOARD on July 29-2025.  I did not need the sound function.

I tried to match the two but the new one is not consistent with the speed.
i.e. with one throttle setting, it changes speed. Where as the older DCC sound value runs  consistent at any speed.  Also the new one does not like to run and slower speed.

I programmed both using the speed table.

Were you aware the decoders in the DCC OnBoard GP40s do not support all the CVs necessary to speed match? Mid and Max voltage settings are the ones not supported. If you can adjust those on the Sound Value GP40 you might try speed matching it to the non sound GP40 instead of the other way around.

I would also add that speed matching needs to be pre-empted by 10 minutes of run time...just to get everything warmed up to operating temps before speed matching ensues

I also agree with jward that the DCC On Board does not allow CV changes for speed - so match the Sound Value to the DCC On Board

For My own DCC On Board purchases I have converted them to full DCC boards that allow all functions

This is so cool, that is, trying to operate two locomotives in a consist mode.   Based on my experience, I too have DCC sound value locomotives, which I found that the new one needed quite a bit of operation prior to settling down to a constant speed. So I suggest that your new locomotive will need some time (in operation) to finally become set and remain at a constant speed. 
As a side note, since you are using them in a consist configuration, to me, that's where all the fun is.  You can take either the first and/or the second engine and manually control their speed as a part of the fun to keep them at an equal stance with one another.  The real fun comes in descending or ascending a grade where one engine may need assistance to support the other.  To me that's a lot of fun keeping pace with the movement of the entire train, and it's an artist that can control both engines, keeping them from jerking along, but produce smooth movement".

Understand that the professional DCC controllers will allow you to do that consisting, or set the two diesel decoders to the same address and/or speed match,  the way the other responders have indicated.   I have the older Bachmann DCC controllers that allowed for two separate controls for each engine. Sorry, Bachmann doesn't make them anymore.  But for me, I have great fun taking a lead engine with several cars, and a second engine in the middle with a trailing set of cars and move them in unison on my layout.  I perform speed matching on the fly. 
I also was told, and I quote:

Quote"Why do all that to get two locomotives to run together when you can program them to do so?  The way you are trying to do this, every time you want to speed up or slow down you have to adjust both locomotives, and until you make those adjustments, they will fight each other.  Can you imagine what would happen on the real railroads if the engineer had to keep running from unit to unit adjusting throttle settings?"

So to me that's where the real fun comes in.  Working to keep the two or even three diesels working together.  But alas, that's me.