Suppose that I have two numpy arrays of the form

x = [[1,2]
     [2,4]
     [3,6]
     [4,NaN]
     [5,10]]

y = [[0,-5]
     [1,0]
     [2,5]
     [5,20]
     [6,25]]

is there an efficient way to merge them such that I have

xmy = [[0, NaN, -5  ]
       [1, 2,    0  ]
       [2, 4,    5  ]
       [3, 6,    NaN]
       [4, NaN,  NaN]
       [5, 10,   20 ]
       [6, NaN,  25 ]

I can implement a simple function using search to find the index but this is not elegant and potentially inefficient for a lot of arrays and large dimensions. Any pointer is appreciated.