Replace two random characters by two other random characters in python

I need change 2 random characters (or more) in a string (x) by other two random characters in other diferent string (y), using Python, example strings:

x=ABCDEF

y=GHIJK

The possible solution would be:

z=ABIDGF

or

z=AKCDEH

All changes completely random. Any help is much appreciated. Thank!