You can do it. Just set the maximum speed on the faster engine to 50%. i don't know what CVs for those digitrax decoders are though. You will have to search, read the manual, or search the internet for that. Buts its called "maximum speed".
What decoders are in these two engines?
I do this in JMRI Deconder Pro, and it doesn't show the CV for that setting. Using this makes speed matching so much easier.