'Twixt is an abbreviation of Betwixt. Milton's word choice describes the hovering angels as not being in any particular location. They are described as nether wholly above, below or surrounding the areas of Hell. The word 'twixt describes the angels as not belonging in any specific place. Milton describes the angels this way because they just fell from heaven, and are still in a position of transition from heavenly creature to minion. The definition of betwixt relate the concept of not completely one thing or another.