With indigo you create a "vat" so all three ingredients are mixed together at once in around 15L of water, and you dip your fibres in. The short answer is that you can indeed use only half of the ingredients in each of the packets and mix it in half the amount of water.
The longer answer is that if you do this, you'll have a fairly small vat, so you'd only be able to dye smaller items, as there needs to be enough water so that your items have a bit of room to move around in the vat. Using only half the indigo also means that your vat will be quite weak, and you will only be able to achieve very pale blues.