How to merge text conversations from multiple numbers into a single one? I have a handful of people who have more than two numbers they contact me from, and I find it annoying to have so many different conversations for one person. I'm looking for a way (preferably, an app) that would let me just have one 'conversation' and just change the number.

I had a phone that did this automatically. I'm 99% sure it was a dual-sim phone, so I'm pretty sure it's possible to do.
To answer your question      


Open Contacts    
Find Contact A's Name.  
Click Edit and add an addition number to contact A, number Two.     
Click Mobile, Home or Work phone.    
Save the contact with multiple Numbers.




It's all pretty straight forward for an experienced human being... :)    


Reloading messages should attach the two conversations as one, if not, future conversations will be marked as the same contact.