Don't know if this will help or not, b/c we are just now doing this ...
I wrote to, too and two on 3 index cards. I put them on the table in front of dd9. Then I made up all sorts of sentences, when she heard "to","too" or "two" she had to touch the index card with the right "to" on it. We are doing this as a drill, b/c I haven't found any other way of doing it.
hth