Merging a Crypt7 and a Crypt8 whatsapp backup
This is how I solved the issue.


backup your existing whatsapp installation 

 adb backup -f whatsapp-new.ab -noapk com.whatsapp

clear whatsapp data by going to settings>apps
upload your old database to /sdcard/WhatsApp/Databases and delete other files in this directory
run whatsapp, restore messages and initialise app (you will have to reenter your phone number; if this doesn't work try installing older version of whatsapp - 2.11.432 worked fine for me)
backup installation with old messages

 adb backup -f whatsapp-old.ab -noapk com.whatsapp

download android backup extractor (https://github.com/nelenkov/android-backup-extractor), and its dependencies (bcprov-jdk15on-151.jar) and compile it (use Eclipse if you are not familiar with this)
extract files from backups

java -cp path-to-libs/bcprov-jdk15on-151.jar:path-to-comiled-abe/bin/ org.nick.abe.Main unpack whatsapp-new.ab wa-new.tar

java -cp path-to-libs/bcprov-jdk15on-151.jar:path-to-comiled-abe/bin/ org.nick.abe.Main unpack whatsapp-old.ab wa-old.tar

tar xvf wa-new.tar
mv apps apps-new 
tar xvf wa-old.tar
mv apps apps-old

copy unencrypted databases to a new directory

mkdir plain
mv apps-old/com.whatsapp/db/msgstore.db plain/msgstore-old.db
mv apps-new/com.whatsapp/db/msgstore.db plain/msgstore-new.db

download sch3m4's wforensic from Github, install it and  follow this https://github.com/sch3m4/wforensic#merging-all-msgstore-files-into-a-single-one (alternatively you could just merge both .db files - they are plain sql3 archives)
clear whatsapp data on phone again; upload the merged database to sdcard/WhatsApp/Databases and name it msgstore.db and delte other database files
repeat step 4 


Now you should be able to see all conversations (hint scroll down to see all conversations - they are not sorted chronologically - but that sorts out with time).