I used ECHOS because I don't want newlines in the console or in the file!
Code:
v:\> do forever ( delay 1 & echos foo | tee v:\foo.txt )
foo
foo
foo
foo
^C
v:\> do forever ( delay 1 & echos foo | tee /a v:\foo.txt )
foo
foo
foo
foo
^C
v:\> type foo.txt
foo
foo
foo
foo
foo