Please show me your shared prong full eternity band that DON'T scratches your e-ring! And let me know why not! Or show me your e-ring setting that will prevent the scratching!
As far as I know, there isnt a shared prong eternity band that will not scratch your engagement ring because with this type of eternity band the girdle of the diamonds are exposed. Also, there isn't a type of engagement ring that will solve this because the eternity band will still be in contact eith the engagement ring at some location, causing damage.
To avoid this you can get a small 1-2mm spacer band to wear I between or consider an eternity ring that is channel set or has the diamonds in a bezel setting.