It's actually pretty simple. Some people like to be very precise and make two batches of soap, but I've found it's much easier to make 1 batch, pour half, add some water to the remaining half, and pour that half.
It is a good idea to do some calculations before hand.
For example, if I enter a batch of 32 oz soap that is 100% lard (to keep this example simple) it says to use 8 to 12 oz liquid. So that means a 1-lb batch (16 oz) would use 4-6 oz of liquid. So I would make a 2 lb batch, pour half of it (eyeballing this) and then blend 1 oz of water into the second half, then pour that.