Good question, I've only used the DME data cards on the Concorde so far, but I'm typing the DME coordinates manually most of the time, on all birds. So I actually never tried the DME data input trough the ADEU on the 707 so far.
Adding the second ADEU reader isn't tricky though. The window will get a bit wider and the second gauge has to be defined and placed.

The Civa manual helps you there and if you struggle, please give me a sign and I'll look into it.  :)