splitting a sparse matrix into two

Question: How can I split 1 sparse matrix into 2, based on the values in a list?

That is, I have a sparse matrix X:

>>print type(X)
<class 'scipy.sparse.csr.csr_matrix'>

that I visualize in my head as a list of lists, to look like this:

>>print X.todense()
[[1,3,4]
 [3,2,2]
 [4,8,1]]

And I have a list y that looks like this:

y = [-1, 
      3, 
     -4]

How can I separate X into two sparse matrices, depending on whether the corresponding value in y is positive or negative? For example, how can I get:

>>print X_pos.todense()
 [[3,2,2]] 
>>print X_neg.todense()
 [[1,3,4]
  [4,8,1]]

The result (X_pos and X_neg) should also be sparse matrices obviously as it's just splitting a sparse matrix to begin with.

Thanks!