Conditional Matrices

This seems like a simple problem but I can't figure it out:

I have two identical DIMENSION matrices.  Both contain only binary values 
NOT identical between matrices.  What I want to do: If in cell (1,1) the 
value in the first matrix (x) equals 1, then I keep the value in cell 
(1,1) in the second matrix (y).  If in cell (1,1) the value in the first 
matrix (x) equals 0, then I change the value in cell (1,1) in the second 
matrix (y)to missing (NA).  Repeat for every pair of cells (coordinates of 
the paired cells always match).

Please help.

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