Athern DD-40 decoder[s]

I have an older Athern 2 motor DD-40. Any ideas on how to install a DCC decoder[s] into it

Yes, I did that several years ago – a TCS T-1 decoder is what I used. I just wired the two motors in parallel. Unless you try to pull a 100 car train up a steep grade, the two motors won’t overload the decoder.