The Dynon EFIS systems don't have a video output. Because of this, there's no way to attach an external screen.
 
Not that I know of....

But, Advanced flight systems does have a very small/thin "slave MFD" available for rear seat installs at a pretty low price point. They are the only ones I know who've done that.

We have done it with a 3rd GRT screen ($2K) in the back of an RV8 as it's only about 2" thick, but the Dynon screens are approaching the point of being too deep to do it. The short answer is Dynon-No, AFS-with a slave MFD and GRT-with another full blown screen (although there are rumors of a slave screen from them as well).

Just my 2 cents as usual.

Cheers,
Stein