At the moment, the Grainfather app doesn't support splitting a batch partway through a brew session. The recipe and brew session are designed to follow a single batch from start to finish.
If you'd like to split a 40 L batch, for example, into two 20 L boils to compare different hop schedules, the easiest approach is to brew as normal through the mash, then manually divide the pre-boil wort into two separate kettles. From there, you can boil each 20 L portion independently with its own hop additions.
We recommend creating two separate 20 L recipes in the Grainfather app (or duplicating your original recipe and adjusting the hop schedules), so you have an accurate log of each hop trial.
This will allow you to compare the hop variations while keeping detailed records of each batch.