How do I prepare the following compound from diphenylmethane?
Here's a mechanism that I thought of (It's more theoretical than practical):



Bromination in the first step will occur at the benzyl position due to a stable free radical. Sodium Propynide then adds more carbon atoms via the SN2 mechanism, which are then reduced with Lindlar's catalyst to give the cis-alkene.

The name of this compound should be (Z)-1,1-diphenylbut-2-ene, but as pointed out in the comments, the preferred IUPAC name (PIN) seems to be (Z)-but-2-ene-1,1-diyldibenzene (verified by ChemDraw)