There are a bunch of factors. There are both correlated and uncorrelated errors that you need to correct. The main source of correlated error is ionospheric distortion, which will affect the two units mostly equally. At the low end (cheap cell phone GPS receivers), the measurement errors unique to each device will tend to dominate.

If you wanted to experiment, there's no need to do it online. If you log the output of the two receivers you can align the timestamps and try different ways of post-processing.