Fair comment, as further research shows that 'Twix' is actually a portmanteau of 'twin sticks'. This means that it's already plural, but that a single bar would be a stick. Therefore 'Twix' is the plural, but half of one should really be called a 'Twick'.
Given that a regular Twix has two bits, but a single Twix when sold in multi packs or miniaturised in Celebrations is also called a Twix, there may be an argument that 'Twix', like 'aircraft' or 'sheep' may remain unchanged in it's plural form.