Yes you can, and no you shouldn't..

- splitting the tablet exactly in half is difficult, especially in the field. And you'll have to take the tablet out of it's package in order to break it.

- a 1 liter water-bottle is cheap..

- Another brand of tablet namely Potable Aqua works with 1 tablet for 500ml of water.

- In an emergency and for short-term, you can use 1 micropur-tablet on half a liter of water.