Is it possible to merge WhatsApp chat histories on 2 Android devices (but 1 account)?
If so, how?
Admission: This doesn't fully answer your question, but it may help others struggling like I was with chat-history transfer. I couldn't better place my tip elsewhere (like here) because I don't have the reputation yet to allow me to do so.
Today I finally (read, much hair pulled over last few days) moved from my old Samsung S4 to a Note Edge (Note 4) with the same phone number. The directions WhatsApp gives for transferring history is incomplete or out-of-date it seems. In my case the last crucial step was to RENAME the msgstore.db.crypt8 file to msgstore.db.crypt. Bingo! This time when installing and activating WhatsApp the missing "Restore" question came up, and all went fine. HTH
Credit: crucial tip discovered in another response here at Stack Exchange.
I'll update this answer once I've tried it.