ammonium tartrate + lead tartrate=? From Googlebooks First Outlines of a Dictionary of Solubilities of Chemical Substances  By Frank Humphreys Storer:


  I find that lead tartrate is dissolved by tartrate of ammonia. 


What is the final product?
The addition of ammonium tartrate turns lead tartrate into a water-soluble tartrato complex: 
\[\ce{Pb(tart) + tart^{2-} &lt;=>> [Pb(tart)2]^2-}\]