I don't think you can do "differential GPS" just with two regular GPS modules. You need a special device
which is in the thousand dollar ballpark rather than the $50 ballpark.
I have done a bunch of experiments with two regular GPS modules to see if I got a more precise location
from them, and the answer seems to me, to be no. You do get some situations where one has a fix
and the other doesn't have a fix, but in terms of precision, not much improvement over the normal
position distribution noise.