Which one of these would be the best one to use for Primo?
maps-buildings-dem-poi-f82/various-dem-files-digital-elevation-maps-for-igo-primo-t14030.html

Thanks.

CGIAR [Premium]/NASA, both are ok

Is it possible to put both of them in the DEM folder or just one?

Only one, and only if your device is not in memory lack.

Be aware that DEM files EAT memory! (and I don't mean on SD card but workspace for the program)
A DEM shows mountains, lakes, etc. It's eye candy, so consider is it that important that the mountain ahead or beside you looks exactly as in reality? Thus the smallest file of the 2 could do.
One or two files? Why should you have 2 DEMs? There is only one world. And I really don't know if iGO8/Primo only uses the latest one (like it does with maps). In the old days I come from, devices didn't have the memory to store a large DEM, then how test 2 DEM? Now I don't care

Nabi is quite correct. Dem files in the US are not recommended. Don't install them.

I have a DEM file in the UK, and I must admit I do like it. I've kind of grown to the eye candy and it always impresses people who are used to garmins and tomtoms and flat maps

Advice will be taken. Thank you all for replying.