Hmmm... difficult question to be honest.
Each work has some easy parts and some more difficult parts and I'll break them down with an attempt to provide a number to your scale... You also need to remember that as a duet the parts might be of different difficulty level. Also keep in mind that my grading will be focused mostly on technical elements rather than musical ones... So I'm taking for granted that if someone can play the parts, then they will also have the musicality required.
1. If only he was carefree
Secondo: Level 3-4. Nothing too serious going on, with plenty of repeated patterns.
Primo: Level 4-5, melody that's in unison most of the time, BUT there are a few 16th notes in thirds, chromatically, which could prove a little more difficult, but again they are repeated. I'd say that there's only one bar to be a bit more problematic.
2. A rather innocent Incubus
Secondo: Level 2-3. Just the bass and basic chords pretty much. The ending is quarter notes.
Primo: Level 3-4. It's got octaves, but nothing else to create an issue
3. How to live in an underground home
Secondo: Level 4-5. Off beat staff, chords in difficult octaves, some melodic elements. Not too tricky I'd say but still...
Primo: Level 4-5. Melodic stuff, some fast triplets 16th notes passages that could need some work, but we're talking about single bars here. Trills and mordents.
4. The sparrow and a rainbow
Secondo: Level 5-6. Probably tricky one, because of the semi-fast 16th notes that need careful planning in the right hand.
Primo: Level 5-6. 7ptets in there, but make total sense. 5plets are more difficult to combine with the other pianist, but work well I think. Melodies in the left hand, with trills on the left.
5. Building light momentum
Secondo: Level 6-. One of the most tricky works. Fast paced, great rhythmic accuracy required, fast passages at hand...
Primo: Level 6-. Same as above
6. Meteor fight
Secondo: Level 7-8. The most difficult work from the set. The whole thing is based on syncopaded chords in the right hand, while the middle part of this work reminds of Ligeti in the right hand, with a very fast ostinato in the left.
Primo: Level 7-8. ditto as above, thick chords, great timing required, a 2 bar passage that needs big hands and fast fingers and 5plets all the way...
This is the most difficult work from the set by far.
7. An unfortunate dialogue, a fair dialogue
Secondo: Level 3-4. Just chords in regular time intervals. NOTHING else going on here.
Primo: Level 6-7. Plenty going on. In fact each pianist have their own title (this is why I put two titles above), and different instructions. The secondo just stays apathetic from anything, while the primo goes overboard with reactions and stuff, with plenty of complicated stuff, basically for the right hand alone...
I don't know if the above help. You can browse some of the pages here: http://www.musica-ferrum.com/catalog/viewitem.php?show=47
(if you click on 'preview' of course) so you should get a better idea of the scores. Also you can log in, with your email (you have to give your email for that, sorry) and get the solo parts, to listen more closely what's going on...