Many properties allow you to pay for accommodation in installments rather than in a single lump sum payment. However, they will usually require you to have a UK-based guarantor (a resident of the United Kingdom between the age of 18 and 75 who will be able to pay your rent).
Yes, many student properties in St Andrews allow for dual occupancy of their rooms (e.g. for couples, best friends, siblings). Some properties offer this option absolutely for free, others charge a little extra. When browsing student rooms on Campusboard watch out for the 'Dual occupancy' label which will explain the terms for that room.