If you cut a key with a key strength of 2^n in half then the key strength of either portion (assumed the attacker acquires the other portion) is reduced to in half to 2^(n/2).   An attacker would be 2^(n/2) times easier.

For 256 bit EDSA the full key has 128 bit strength security a half key then only has 64 bit strength if the attacker has the other portion.  While it may take some time and effort that is computationally feasible.  64 bit key strength should not be considered safe from brute force attack.  You have made the attackers job 2^64 = 18,446,744,073,709,551,616 easier. 

A much better option would be to XOR two 256 bit "partial keys" to create the final key.  Each partial key retains the full key strength.